My seat bag contains the usual patch kit, tube, tire levers, etc. It has a couple extras that I've found to come in handy:
1) a couple pairs of latex gloves -- keep your hands clean when doing any repairs
2) A sample pack of baby wipes -- good for post-repair cleanup, also for treating scrapes, etc.
3) Zip ties in a couple sizes for emergency repairs.
Having a couple $$$ in there serves 2 purposes: to spend if you need it, and you can use a dollar bill as an emergency boot for repairing a tire. If you've got room left over you can stick in an energy bar or two.
That will set you up pretty well for anything you're likely to encounter on the bike trail.