I would just do a cook out or bonfire (not sure where you live). I turn 21 in July and we are having a bonfire/cookout. I also find out what I'm having right after my birthday so we are going to wait and do gender reveal at the same party!
Water park? Beach or lake if its not cold? Movie with friends? U can do pretty much anything that doesnt involve alcohol.. maybe no tattoos either. Ive gotten a couple peircings since i turned 18 (i was 6 days pregnant on my bday and didnt know). It can still be fun!