Including what I normally keep in my pockets (3 1/2 in folding knife, pack of gum, wallet, 2 pens, keys and a folded up piece of paper)
and what I'm wearing (pants, 2 T-shirts, belt, shoes, socks, underwear, watch and baseball cap.)...
I would jam my backpack with one change of clothes consisting of an extra pair of comfortable shoes, 2 T-shirts (I always wear 2, so that's one change.), and fresh underwear. Additionally, I would pack as many pairs of socks as I could dig up that have no holes, haven't worn thing, etc. I'd probably take a hoodie too, because those are usually enough to keep me warm in the coldest days of winter, and it would make a great pillow too. I would throw in anything small I have that could be sold for an amount of money worth lugging it around until I do sell it. A stick of deodorant, a bar of soap and some scissors to keep my hair short so washing it wouldn't be a problem, because I don't want to be a smelly bum. I'd probably take a few bottles for water, a couple bags of trail mix, a package of multi-colored sharpie markers, colored pencils (with sharpener) and a sketch book. It would be interesting to sit and draw the various things you see from a different perspective than you are used to. Then you could probably sell the pictures to people who pity you. Last but not least, I would take my old boy scout handbook that has been in my closet for a few years, for one it would give me some fair reading material, and two, it may be helpful.
That would probably stuff a backpack plenty full but I'm sure it would thin out after a few weeks until I get the rythmn of bummin' down.
Before I got too dirty and disgusting, I would work on getting a low-class job to try and make some money. A little money goes a long way when you don't have bills.
Edit: Then I'd head for the mountains and look for JemCam, because he clearly knows how to get by and I'm sure he'd appreciate some company