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!
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!