This may not be your cup of tea, but I find it works for most three (or at most four) day uses.
http://tacspec.com/personal/back-pack/
Although it is a military-designed bag, it functions really well as a daybag/weekend bag. It's got a ton of small pockets that seal up tight, with a good sized main compartment that will hold a weekend's worth of clothes. There is a loose divider in the main compartment so you can separate dirty from clean.
In the summer, when I travel in shorts and flip-flops, this is the only bag I use, even for a three-week trip. It's compact, and conforms to my back, so getting on and off crowded buses and planes is easier. It's small enough to carry on, so no checking in luggage, which is a must for me. It's got a detachable waist belt, and tons of loops from which you can tie almost any additional items. It's also padded, very comfortable to carry, and has a pouch for a Camelbak, etc.
They're a little expensive, but I got mine used on ebay for $100. Check around.