A magnifying glass ain't a bad idea, either. Just a small one. The kind you might find on a swiss-army watch. Those CCG's and atlases can get damn hard to read.
Also, if you can find one, some headshops sell lock-up metal dugouts. They're little metal boxes that descreetly hold a reasonable amount of weed, and a tiny one-hitter. It locks up when you close it, and you can only open it by hitting a series of "sweet spots" on the box, which will unlock it. It's a good place to keep yer stash, it doesn't smell, and if cops can't figure out how to open it or what it is, they can't bust you for it.
I'm sure this has already been mentioned (I might have even already said it), but bring a fucking deck of cards. I don't even care if they're Magic: The Gathering cards. They will get you through hours of boredom. Even if its just solitaire or go-fish. It beats sitting on your ass tossing pebbles off a bridge.
If you have a refillable lighter, bring it. It doesn't need to be some fancy Zippo, just a refillable lighter. Bic's are easily lost, and quickly drained of fluid, and matches tend to be troublesome.