For a female or a male?
Do something fun like ice skating, roller skating, bowling or miniature golf! When I was in high school and just graduated, we liked doing progressive dinners. For girls, book a mini spa day or mani/pedi party. How a about a dinner and movie?
I think choosing an activity that makes a memory is a lovely idea - perhaps doing something they've never tried before :)
Hi Maria! There are a lot of fun things you can do for an 18th birthday!!
If the soon to be eighteen year old has a best friend or two that you know pretty well or you know the parents, call then when you know they are not together. Talk to the friends about what they think the person would be interested in doing or a theme for the party and who to invite if you are planning on having some of the persons friends included. If you have to call the parents and let them know what you are doing and ask them to talk to the friends to help get ideas from them without letting the birthday person know what is going on. You can also get ideas for what to get for the birthday person that they really want for gifts.