easy to regret:
not taking a knife.
not taking a spare flashlight, plus batteries and spare bulb (if not an LED light).
not taking bug repellent and sunscreen.
Personally I like those little "sample size" ones with the carabiners attached.
Also, hand sanitizer in the same sort of package.
not having along a compact but thorough first-aid kid -- alcohol pads, band-aids, antibiotic/painkiller ointment or cream, a couple ace bandages in case of sprains, some after-bite or sting-stick, and some aloe vera and caladryl in case of sunburn / poison ivy. I'd stick a couple needles, a good bright small specific-to-the-kit flashlight and a pair of tweezers in, along with a package of dental floss. I always used to include telephone money, but you might want to add one of those hand-turned cell-phone battery rechargers to prevent being rendered incommunicado at an inconvenient time.
what are you planning for activities? hiking? biking? birding? starwatching?
those things'll dictate part of your load (clothes, boots, binocs, a journal and /or guide book(s), water bottle, snacks). Remember: stuff always takes up more room than you thought it would, and dirty laundry never packs down as small as clean.
have a good book and a couple simple games (checkers or yahoo, for instance) and a deck of cards in case the weather turns to hammered pig snot on you.
There's NO SUCH THING as too many shoestrings. You can take a shoestring and tie a couple loops in it, and voila: you have a shoulder strap for your water bottle.
(You do have your water bottles wrapped in clean orphaned tube socks to keep the water cool and the bottles from sweating, right?)
I'm thinking in terms of stuff that's easy to do and won't cost a ton, here ... you probably already have good shoes, bikes, maybe even binocs. The library can provide you with guidebooks and reading material.
you don't have to buy a lot of gear. take a spoon apiece and an unbreakable cup, and you're fixed for eating and drinking. Of course, you'll want soap and a dishtowel to clean your dishes (and something to heat water in for coffee / tea / oatmeal / soup and dishwashing).