Everyone has a different idea of what they want a backpack to do, or what exactly they carry on a overnight trip. Winter or summer? I tend to be minimalist. I don't take much on an overnighter, I want my pack superlight. I really like Arc'teryx packs, they are exceptionally well built, but usually expensive. Often times you can find sales. Check Mountain Gear (mgear.com) in the sale section. All the packs I've seen in the in the sub $50 (retail) range are usually not worth the money. I like climbing specific packs because they are lighter. This one looks nice and is on sale.
A good size pack is from 40 - 80 liters, depending on what you plan to do and what climate. If it's just a mile walk to the site I'd go with the 40 to 50 liter pack. If you are hiking in to a site that is primitive with a 65 liter on up.
Also, do you want an inner or outer frame?
Heads up, get fitted for the pack by someone who knows what they are talking about.
What all are we talking clothes + bathroom stuff + computer...or what?
I use the North Face Hot Shot as my student back pack and its pretty good, comfortable, not to big or to small, and has a computer pouch in the back. I actually have the grey one pictured. http://www.thenorthface.com/catalog/sc-gear/hot-shot_3.html
also, it REALLY depends on what you're after. for all of my hiking and backpacking, I prefer tactical bags.
http://www.511tactical.com/All-Products/Bags-Backpacks/Backpacks/RUSH-72-Backpack.html here's a link to my current favorite, I can drop 2x 100' rolls of rope into the bag, 5 MRE's, 2 rappelling harnesses, 2 chalk bags, 2 3L camelbacks, a large collection of carabiners and descenders, a large bag of trail mix, and 2 changes of clothes. My backpack straps to the top, the bottom, or the back, wherever I want to put it. I could easily strap a small tent to it as well. In addition, I could fit up to a 17" laptop (or a ballistic plate) in the back as well...
they ride nicely on your back and are reasonably small, with no hard frame at all, easy to fly it if you need to, in fact, I use it as my carry on whenever I fly.
hmm....if this is true I'll consider getting one for hiking, it'd be easily half the weight of my 72 hr assault pack I normally carry....but, then again, I don't worry about ANYTHING hurting the assault pack....
I bought an Outdoor Products Stargazer internal frame pack from Wal-Mart for like $52 for an overnight trip up to the Fiery Gizzard, and it was perfect for what I needed. I didn't care to spend $150+ for something that I wouldn't be using too often