40L is pretty huge for his contemplated use.
What style do you want?
Honestly, it's hard to beat Jansport or LL Bean for value. Both have lifetime warranties where they'll take back stuff that's basically just worn out. (I would never ask for a new backpack just because I put ten good years into one and it finally gave out, but it's nice to know they stand behind their products.) I used a Jansport Superbreak through high school and college. It cost about $30. I probably still have it somewhere. I like that it's unstructured. It holds a surprising amount when it's stuffed full, but I also used it traveling with nothing but a book, a camera, a water bottle, and a windbreaker, and it didn't look like it was comically under-filled.
Osprey makes great hiking bags that are very light and distribute weight well, but they look a little techy and they won't last as long as a school-style backpack.
If you want something higher end, but not luxury, you can't go wrong with Tom Bihn. Check out the Synapse, which comes in a couple sizes.
If you want something that says, "I'm an athlete," most running brands have some kind of backpack, Nike and New Balance in particular.
Topo Designs also makes decent stuff, but a little too mountain-hipster for my taste.
If you want a design that's really optimized for travel, check out clamshell packs. Patagonia, Timbuk2, and Cotopaxi all make them.