… the first commandment of adventurers everywhere, "Thou shalt not travel with anything thou cannot carry at a dead run for half a mile and store under thy seat."
What To Pack It In
Leisure Travel Bags
Casual/tourism travel typically involves more (and longer-distance) carrying of one's bag over more varied terrain, including stairs, public transportation, cobblestones (and in case you hadn't noticed, urban streets are becoming more cobblestone-like), and good old terra firma (and not so firma).
So look for a true backpack-style suspension system, one that incorporates internal stays (for vertical support) and a padded hip belt, because the weight is supposed to be on your pelvis, not on your shoulders. To get this, you will likely have to sacrifice a bit of packing ease: typically, the bag will have one large main compartment, and it may not open completely flat, though it should definitely have internal tie-down straps. External (compression) straps are a helpful feature, particularly in a larger-sized bag, both to reduce the size of the (packed) bag and to stop items from shifting about.
A variety of factors — both technical (e.g., they generally open only at the top, which makes packing difficult) and social (e.g., backpackers are perceived as undesirable in some areas) — tend to preclude the use of "real" backpacks, so you need something that can be made not to look like one! This style of bag, in which the shoulder harness and hip belt can be hidden away (typically behind a zippered panel), has come to be known as a "travel pack".
Alas, true travel packs have become something of an endangered species of late. There is no shortage of real backpacks, or of bags with hideaway shoulder straps (of varying quality), but the notion of a soft, suitcase-styled bag that incorporates a true suspension system (as outlined above) seems to have fallen from favour, at least with those companies that offer large-scale distribution. It's a shame, really, and a bit hard to explain, given that there are a lot more leisure travellers than mountain-climbers!
My clear choice in this category is the MEI "Voyageur" pack (pictured at right), which is the maximum permitted carry-on size of 22×14×9 inches (56×36×23cm), made of 1000 denier Cordura with an interior urethane coating, and incorporates parallel (removable) internal aluminum stays; it weighs 3.5 pounds (1.59kg).
The hipbelt and shoulder harness are lined with nylon knit, and padded with closed cell foam; these zip out of sight for carry-on use, and a more "luggage-like" appearance. Complete with lockable YKK zippers (with weather flaps), internal tie-downs, and both sternum and compression straps, it comes in a wide variety of colours, with a suggested retail price of USD$136. MEI products are fully warranted for the life of the purchaser. Curiously, this bag was the first of what are now called "travel packs", and is arguably still the best; it has enjoyed a variety of modest updates and improvements since its introduction in 1978, but the (clean, rectilinear) design remains fundamentally unchanged.
Quality will out.
Those on a more limited budget might find an appealing alternative in MEI's "Convertible" (shown below), which sells for USD$100. It's constructed using a combination of 420 denier ripstop nylon and 1000 denier Cordura, and measures very slightly smaller — 22×13×9 inches (56×33×23cm) — than the aforementioned "Voyageur" (not a bad thing, in my view), yielding a modest weight of only 2.75 pounds (1.25kg). The bag lacks the strict rectilinearity of its larger brother, along with the aluminum stays, external compression straps, and padded hipbelt, though it compensates for the latter somewhat with a decent nylon webbing waist strap, certainly a useful feature.
Leisure/adventure travellers who are willing to sacrifice some carrying comfort and capacity for improved storage organization might also consider dual-purpose travel bags.