If you haven't been paintballing then you are missing out my friend; paintball is THE social outdoor experience. It's perfect for birthdays, and makes a great unique corporate or anniversary event. It's a little expensive, but it's the experience is well worth it. I highly recommend it to everyone; do this instead of bowling or going to the movies, or even spending the night out at a bar or club.
Paintball is also a great workout since you have to run around and crouch and carry your paintball gun everywhere with you. It feels like you're in a game of CS:GO or Call of Duty.