Question: fun things to do on your eighteenth birthday?
Answer: You're an adult! Go out for brunch. ... 
Do some taxes. Talk about the weather. ... 
Or build a pillow fort and have a movie marathon. Apparently that's a very grown up thing too… ... 
Book an outdoor activities session, like zorbing or paintballing. ... 
Have a spa day. ... 
Swim in the sea, if it's safe. ... 
Hire a limo. ... 
Throw a themed party.