Most important, although unused so far (thankfully!): Epi-pen, (Bee sting allergies are a bitch) a hockey puck to knock down discs, and some handy poison ivy wipes.
Important stuff I've actually used: really only water. For summer I'll put my Birdie Bag back in.
While I agree with a lot of what guys have mentioned I tend to prefer to keep most of that stuff in my trunk rather than actually in my bag.