Casual/tourism travel typically involves more (and longer-distance) carrying of one's bag over more varied terrain, including stairs, public transportation, cobblestone streets (and in case you hadn't noticed, many 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.
It should be possible to lock the bag securely (usually via one or more padlocks through the zippers, the better of which are specifically designed for this purpose). It should not have outside pockets (unless you care to provide both amusement and revenue to the folks who ride up on top of the bus with your luggage).
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!
Recommended Leisure Bags (Travel Packs).
My current choice in this category is the MEI "Voyageur" pack (pictured at right), which is 22×14×9 inches (maximum permitted carry-on size), made of 1000 denier Cordura with an interior urethane coating, and has a parallel stay internal aluminum frame; 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 YKK lockable zippers, tie-downs, and compression straps, it comes in a wide variety of colours, with a suggested retail price of $136. MEI products are 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 small improvements since its introduction in 1978, but the (clean, rectilinear) design remains fundamentally unchanged.
Those on a more limited budget might find an appealing alternative in MEI's "Convertible" (shown at left), made with a combination of 420 denier ripstop nylon and 1000 denier Cordura, and selling for under $100. Only slightly smaller than the aforementioned "Voyageur", it's 22×13×9 inches, but lacks external compression straps and (unfortunately) a hipbelt.
Some of the very best products are simply not available at your local shopping mall. This is often because they are made by companies that eschew large distribution networks and sales intermediaries (and their associated markups) in favour of higher quality manufacturing (and better warranties). Fortunately, the Internet has made the worldwide availability of such items much easier. So, where possible, I provide contact information for many of the products & manufacturers mentioned on this site; you'll find this information on the Suppliers page.
Leisure/adventure travellers who are willing to sacrifice some carrying comfort and capacity for improved storage organization might also consider dual-purpose travel bags.
Whatever type of bag you choose (but especially a travel pack), consider tucking in a lightweight, highly-compressible daypack. 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. Many business travellers find such a bag useful as well: it can, for example, provide a convenient place to keep items you will use in-flight separate from everything else, allowing you to quickly remove them from your main bag before storing the latter in the overhead compartment. When not in use, a daypack of the type I recommend takes up very little space in one's bag (the more compressible bags sometimes come with tiny stuff pouches, though I prefer to simply pack them flat when not in use, thereby reducing hassle and saving weight).
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 cavernous space.
An excellent offering is Kiva Designs' "Rick Steves Civita Day Bag" (shown at right), which comes in several colours and sells for under $20. Obviously designed by someone with plenty of real-world travel experience, it collapses to almost nothing, yet provides over 900 cubic inches (15L) of storage space in three differently-sized compartments, surrounded by a soft microfiber fabric that allows it to be used as a pillow when stuffed with a jacket or sweater. With its lightly padded shoulder straps and two mesh bottle holders, it measures 13×10×7 inches, and weighs 8.6 oz. (244g).
Those wanting greater carrying capacity should consider Barefoot Enterprises' "Wanderlite Packable Daypack" (pictured at left), which sells for under $28. Its spartan teardrop-shaped design encloses — primarily in a single large compartment — a generous 1925 cubic inches (32L) of storage space (18×14×8 inches, 46×36×20cm), yet weighs only 8 oz. (227g). The 420 denier nylon packcloth fabric is somewhat stronger and more weatherproof than the Civita's, though is partnered with lower quality zippers. Its wide shoulder straps are unpadded, though surprisingly comfortable.
Yet another option is the Eagle Creek "Packable Daypack", which comes in a choice of red or black micro-ripstop nylon, and lists for $30. Its midrange dimensions of 15.5×11×4.5 inches (39×28×11cm) provide 1250 cubic inches (20L) of storage, but a 10 oz. (280g) weight definitely relegates it to the "third choice" category.
Whenever you have occasion to temporarily remove your daypack, and place it on the ground (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).
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.