I just went out to town with my best friend on my 18th birthday and nothing else. I don't know what kind of people your friends are, but I don't consider it weird to have a small celebration.
edit: Of course, that might not help with the not feeling like a loser thing. Getting rid of insecurity can take time, but there is a saying I like: "you can't be made a victim unless you feel like one". If you think going go-carting will be fun, and you don't want anything else than you have no reason to feel bad about it. As long as you're having fun, you're winning.... Just like Charlie Sheen is still winning
http://www.youtube.com/watch?v=pipTwjwrQYQ