So I'm really just getting into using a backpack. But even after browsing around here, I realized that I still don't have much of an idea about how to do it properly for just going around town.
In general....
Spend enough for quality. Most cheaper backpacks are poorly made ripoff versions of real gear. Real gear costs money because it was designed by people who know what they're doing, made by people who know what they are doing, and backed up with a warranty. Maxpedition, Camelbak, SpecOps, ATS, Eagle, TAD, and others all make end-of-the-world ready gear that is far more enjoyable to use.
For a daypack, I never use one with a hip belt or overly padded straps. If you are loading up with so much weight that you need a hip belt or a lot of padding, just to use 'around town', you're doing it wrong. Most "hip belts" found on daypacks are just a non-padded piece of webbing, which does absolutely nothing to transfer any load to the hips, it's just another strap to be annoying and get in the way.
I avoid MOLLE webbing covered 'packs, they attract attention, add weight, and get caught on anything you walk past.
Another question is just what a backpack should contain if it's ultimately left in the vehicle for when going out and about. If the overall weight isn't significant, should it be packed to work in conjunction with the EDC gear one carries, or should it be used for storing replacement important items should one lose them? Like knife, flashlight and multi-tool?
EDC stuff can get lost or stolen, it never hurts to have a spare knife, flashlight, or whatever else goes in your pockets sitting ready in your 'pack. I always have a folding knife and a AAA flashlight clipped to my pockets, but still keep a spare blade (Leatherman Skeletool makes a great backup knife) and flashlight (Surefire EB1T) in my 'pack.
Here's my spare light and cutter, in a Maxpedition holder doohickey stuck to the flap of my small daypack:
My EDC backpack, that goes to work or on dayhikes or wherever I go, is small. It's an ATS 2.6 two tone that doesn't stand out in a crowd, is well designed and highly functional, and is on the small side. It's just big enough for a 1L Nalgene, lifestraw, hat, gloves, spare underwear/socks/t-shirt, sweater, snacks to last the day, and a trauma kit. It has shock cord on the outside to lash a jacket, and two rows of molle webbing on the bottom (where it isn't visible) to tie a sleeping pad or bag or whatever.
My lolsurvival gear fits into a Maxpedition Fatty Pocket Organizer. While everything in there gets used from time to time, none of it is constantly required. Most of it is pretty self-explanatory, except for the green match case which contains a pretty decent sewing kit. What is not shown in the picture is the contents of a pocket first aid kit in the zippered pocket on the right hand side, and the emergency pocket poncho on the left side. Here's the Fatty Organizer:
Hope that this helps!