-Mosquito repellent

I also like a lot of 'non-essentials' to keep me occupied during rainy days; otherwise, everything's already been mentioned that I bring, and then some:

-Books (not novels, but books that you can work with, that take longer to get through, and that will improve you somehow: Foreign language books, chess books, bicycle repair books, books on nature/field guides, outdoor survival manuals, etc...)
-a small pair of binoculars for birdwatching, along with a camera & zoom lens.
-A small musical instrument and some blank staff paper.
-Mini sketchpad/notebook and pencil.
-I also like the ThermaRest chair/pad combo, and will frequently turn away park picnic tables in favor of it.
-LED headlamp (instead of a flashlight; attaches like a sweat band and can be used for everything from a night light for reading to a headlight for your bike)