Every one-bag traveler eventually needs a second bag, because a 40L pack is not something you want on your back while walking round a city, and leaving it at the accommodation is the entire point. The question is what that second bag should be — and these two represent the extremes of the answer.
The Osprey Daylite Expandable 26+6 is a real bag: 26 liters, structured, with a warranty, that expands to 32L and can serve as your personal item on the plane. The Sea to Summit Ultra-Sil Day Pack is 2.5 ounces of siliconized nylon that stuffs into its own pocket at roughly the size of a lemon and costs you nothing to bring.
At a glance
Osprey Daylite Expandable 26+6 versus Sea to Summit Ultra-Sil Day Pack 20L| | Osprey Daylite Expandable 26+6 | Sea to Summit Ultra-Sil 20L |
|---|
| Capacity | 26 L, expanding to 32 L | 20 L |
| Empty weight | Not published | 0.16 lb (2.5 oz / 72 g) |
| Packs down to | No — it holds its shape | About 2 x 4 x 7 in |
| Structure | Padded back panel and straps | None — unpadded, unframed |
| Laptop | Not published | No sleeve |
| Water resistance | Coated shell | Water-resistant only — seams unsealed |
| Warranty | All Mighty Guarantee | Not published |
| Our carry-on read | Convertible / expandable | Personal-item sized |
The real difference: does it have to carry weight?
Everything else follows from this. The Daylite has a padded back panel and padded straps, so it carries a laptop, a couple of liters of water, a jacket and a day’s shopping without complaint. The Ultra-Sil has thin webbing straps and one layer of fabric between the contents and your back, so a hard-edged object inside it is an object you feel.
The practical threshold is about 10 lb. Under it, the packable is genuinely pleasant and its weightlessness is a superpower. Over it, the straps bite and the load sags, and you spend the day adjusting it. Nobody should carry a laptop in a packable daypack, and Sea to Summit does not suggest you should.
The one you can fly with as a personal item
This is the Daylite’s strongest and least-discussed advantage. At 26L, a soft pack generally sits inside most airlines’ personal-item allowances, which means it goes under the seat in front of you, never gets gate-checked, and never faces a sizer.
That matters more than it sounds. A bag that stays with you is where your passport, medication, laptop and power bank belong — power banks in particular are prohibited in checked baggage, so a bag that cannot be taken from you is the right home for them. The reasoning is worked through in personal item vs carry-on and power banks on planes.
The catch is the expansion: at 32L the Daylite is no longer a reliable personal-item bet. Fly it zipped down. Expanding a bag at a gate is precisely when you least want to look larger than you are.
The one that costs nothing to bring
The Ultra-Sil’s argument is that it removes the decision. It weighs 2.5 oz and packs to roughly 2 x 4 x 7 inches, so it lives permanently in a pocket of your main bag and you stop thinking about whether to bring a second bag at all.
That is worth more than its capacity suggests, because the times you want a spare bag are rarely the times you planned for: an unexpected market, a beach afternoon, a supermarket run for a self-catering flat, a day where the weather turns and you need somewhere to put a jacket. Twenty liters covers all of those, and none of them warranted carrying a structured pack across a continent.
Be clear on the waterproofing, though. Sea to Summit states that the fabric is waterproof but the pack is not — unsealed seams and an exposed zip. It sheds a shower; it will not keep a camera dry in real rain. Their Dry Day Pack, with taped seams and a roll-top, is the version that does, and the distinction is the whole subject of our waterproof roundup.
Durability, honestly
The Daylite is built to be used daily for years and is covered by Osprey’s All Mighty Guarantee, which repairs any Osprey product from any era. The Ultra-Sil is deliberately fragile: ultralight silnylon does not tolerate being dragged over stone, over-stuffed with hard-edged objects, or used as a laundry bag on a rough floor. Treated as what it is, it lasts for years; treated as a duffel, it lasts a season.
That is not a defect. It is the trade you make for 2.5 ounces, and Sea to Summit publishes the numbers that let you understand the trade — which is why it is in our registry at all.
The picks
#1Best if it's your personal item
Osprey Daylite Expandable 26+6 Travel Pack
The daypack that does a second job. 26 liters is a genuine day bag — city walking, a laptop, a jacket — and the expansion adds six more when the flight home has to swallow what the trip accumulated. It's the one pack here we'd happily use as a personal item under the seat and as the day bag once you land.
Strengths
- +26L handles a real day out, not just a water bottle and a wallet
- +The +6L expansion is there for the trip home, then zips away
- +Osprey's All Mighty Guarantee, which almost nothing else at daypack money carries
Trade-offs
- −Expanded, it stops being reliably personal-item sized — zip it down to fly
- −Costs several times what a packable stuff-sack daypack does
| Capacity | 26 L |
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| Weight (empty) | Not published |
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| External dimensions | Not published |
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| Carry-on fit (our read) | Convertible / expandable |
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| Laptop sleeve | Not published |
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| Material | Not published |
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| Warranty | Osprey All Mighty Guarantee |
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Fit note. The '26+6' is the whole spec. Fly it at 26L, where a soft pack of that size sits inside most personal-item allowances, and treat the extra 6L as an at-destination feature — expanding it at the gate is exactly when you least want to be measured.
Specs read from the product listing, on August 11, 2026. “Not published” means the maker does not state that figure.
#2Best if it's a spare
Sea to Summit Ultra-Sil Ultralight Day Pack 20L
Twenty liters that weigh two and a half ounces and stuff into their own pocket at roughly the size of a lemon. This is the second bag you carry because you can't feel it — a beach, a market, a day hike — and it is emphatically not a bag to carry a laptop or a heavy load in.
Strengths
- +2.5 oz and packs to about 2 x 4 x 7 inches — it disappears in a main bag
- +20L is real capacity, not a token amount
- +Simple, durable siliconized nylon with nothing to break
Trade-offs
- −No padding, no frame, no laptop sleeve — thin straps bite under weight
- −Water-resistant only: unsealed seams and an exposed zipper let water in
| Capacity | 20 L |
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| Weight (empty) | 0.16 lb |
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| External dimensions | Not published |
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| Carry-on fit (our read) | Personal-item sized |
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| Laptop sleeve | Not published |
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| Material | Ultra-Sil siliconized nylon |
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| Warranty | Not published |
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Fit note. Sea to Summit publishes the numbers that matter for a packable: 20L capacity, 2.5 oz (72 g) empty, and a packed size of about 2 x 4 x 7 in. They also state plainly that the fabric is waterproof but the pack is not — the seams aren't sealed and the zipper isn't covered. That honesty is why it's here.
Specs read from the product listing, on August 11, 2026. “Not published” means the maker does not state that figure.
The honest verdict
Buy the Osprey Daylite Expandable 26+6if the second bag is doing real work — if it is your personal item on the plane, if it carries a laptop, or if you want one bag that covers day trips and commuting at home. It is the only one of the two you could genuinely travel out of, and the +6 is a well-judged answer to the flight home.
Buy the Sea to Summit Ultra-Silif the second bag is a contingency — beaches, markets, an unplanned day out — and your main pack is already doing the carrying. At 2.5 oz there is no argument against having one, which is why it is the item we recommend to more travelers than any other on this site.
And the setup most experienced one-baggers actually run: a structured day bag as the personal item, plus a packable stuffed in a pocket. The combined weight penalty is a few ounces, the combined cost is modest, and it covers every case either bag misses. The full field is in our travel daypacks roundup.