Pack less, and become unattached to possessions.
And then … pack less.
What To Pack It In
Day Travel Bags
Whatever type of primary bag you choose, consider tucking in a lightweight, highly-compressible day travel bag (daypack). On leisure travels, this gives you the ability to leave the main bag at your hotel, B&B, pension, or station locker, and carry your around-town necessities (rain jacket, sweater, water bottle, guidebooks, etc.) in the daypack.
A daypack can also serve the function of "last-minute" duffel bag. Understand, though, that while most North American airlines permit a "personal" item (e.g., a modestly-sized bag) in addition to your carry-on, this is not true of most of the world, where it's "one bag" only. So be prepared to check your main bag when carrying the emergency one separately.
Daypacks tend to be a more personal choice than other bags, thus it's not as easy to suggest an "ideal" solution. Some people want an unobtrusive bag with modest carrying capacity, while others prefer to keep their options open with a more generous amount of storage space. Some want lots of compartments and bottle holders, while others crave a single unconstrained space.
Recommended Day Travel Bags
In their attempts to meet the dual goals of compressibility and reduced weight, the bags I suggest here are not as rugged as many that you can buy, or festooned with as many "features" (neither are they as expensive), though they are surprisingly strong for their weight. Unlike the main bags I recommend, they are unlikely to last a lifetime, though will provide many years of valuable service for those who travel lightly.
There's little argument as to the lightweight, minimalist winner in this category: "Sea to Summit's Ultra-Sil® Day Pack". At under USD$33, measuring 19×11×8 inches (48×28×20cm), and weighing a truly miniscule 2.4 oz. (68g), it provides a capacious 1220 cubic inches (20L) of storage in its large, single, two-way zippered compartment, and yet collapses into an egg-sized ball that stuffs into an attached sack, and can be fastened to your key-ring! It achieves this minor miracle through the use of a high-tech, super-thin, siliconized Cordura nylon fabric that is surprisingly strong (though not puncture-proof). With its bar-tacked reinforced stitching at stress points (the bag has been stress tested to 350 lbs / 159kg, though you'll likely want to keep the weight under 10 lbs / 4.5kg), this is the day travel bag that I carry in my main bag.
Many travellers prefer a cylindrical style day bag, despite what some consider a somewhat unconventional appearance. My favourite in this category is the under-USD$20 "Ultralight Travel Daypack", from Hammock Bliss. Measuring 20×11×7 inches (51×28×18cm) and weighing 6.5 oz. (184g), it provides an unobstructed 1100 cubic inches (18L) of storage in its top-loading main compartment, and also includes a zippered inside pocket for small items, plus two mesh side pockets (visible in the photo at left); the right-side pocket incorporates a top closure, allowing it to double as a storage pouch for the bag (simply turn the pocket inside out with the bag inside). Lastly, and importantly, it features well-designed, comfortable, double-padded shoulder straps that incorporate a carry handle.
REI also makes a couple of very nice packs in this style (albeit more than 50% heavier): their "Flash 18" and "Stuff Travel" models.
An offering along more familiar lines, and one having benefitted from several revisions over the years, is the "Eagle Creek Packable Daypack" (pictured at right), which retails for around USD$28 and also comes in a choice of colours. Its 150 denier ripstop fabric encloses a decent 975 cubic inches (16L) of volume in a single main compartment; a flat front pocket with a reversible zipper doubles as a storage pocket, in much the same fashion as the Hammock Bliss bag, above. The 6 oz. (170g) bag sports breathable mesh shoulder straps and a key fob, but no internal or water bottle pockets.
A final commendable design worthy of mention is Eddie Bauer's "Stowaway Packable Daypack" (shown at left), available in several colours for USD$30 or less. At 11 oz. (312g), it's a little heavier than I like to pack in my main bag, but I have found this useful as my only bag for short, casual-dress trips (that do not require bundle-wrapped clothing and such), and situations with extremely restrictive airline carry-on policies. This is also the bag I use when at home, as my regular "around town" bag. Collapsing into its own zippered stuff pocket, it provides 1220 cubic inches (20L) of storage space in three differently-sized compartments, plus a velcro-tabbed internal panel to protect paperwork/books and the like. With its lightly padded shoulder straps, two mesh bottle holders, and four D-rings for attaching gear, it measures 18×10×9 inches (46×25×23cm).
Whenever you have occasion to temporarily remove your daypack, and place it on the floor (at a restaurant, say), a good habit to cultivate is passing your leg through one of the shoulder strap loops. Not only will this hamper anyone trying to grab the bag, it will also remind you not to leave it behind when you move on (this being the primary cause of lost packs). Don't do this on an airplane, though: it will impede your exit in an emergency situation.
A locking mountaineering carabiner can also be employed to temporarily secure a day bag.
Finally, a daypack also makes a good emergency bag (as an alternative, or even addition, to a small duffel), useful if your return voyage finds you with considerably more stuff than you started with. Don't forget, though, that mailing items home can often rescue you from this problem.
A “Stealthy” Alternative
Some travellers dislike — for a variety of reasons — carrying a day bag. An attractive alternative, especially in recent years, is the use of what I term "stealth" clothing. Such garments incorporate in their design a copious number of pockets, often content-specific, that are largely hidden from view (unlike the classic "safari/photojournalism vest", a much-less-stylish version of this approach). American manufacturer ScotteVest (SeV) dominates the market, coupling a broad array of attractive products with very successful marketing.
All manner of clothing (from underwear to expedition coats) can be had in stealth versions. Special pockets for cellular phones, music players, tablets, and the like — even SeV's patented "Personal Area Network" (built-in support for earphones and their accompanying wiring) — will be of particular appeal to those who like to carry lots of electronic items. And even the more minimalistic globetrotter will enjoy the convenience of protected storage for — yet instant access to — money, camera, passport, guidebook, and such.
Outer garments provide the most storage, thus offer the greatest stealth potential (when climates permit). Carrying cellular phones and the like in this fashion also means that, because they remain securely in your jacket, they're less likely to be counted among the lost when going through airport security (each large airport reportedly collects about a thousand such left-behind devices annually). It also makes your valuables less accessible to pickpockets.
And those having trouble with restrictive carry-on weight limits will find stealth clothing to be ideal for reapportioning weight!