I don't remember my 18th birthday. It was only 4 years ago, and I didn't drink back then, but for some reason I can't remember a thing. It must have been boring, and that makes me sad.
Normally I would recommend hookers and blow, but those are two things on your "no-no" list.
The 'no drinking' thing isn't too big a deal, but I can guarantee someone at your party is gonna have a bottle of something.
If you're going to to the whole 'big field/bonfire' thing, I recommend bringing some cool shit to burn. Examples:
One of those giant wooden McGruff the Crime Dog signs - This can be found at your local park, and dug out in about an hour.
Something from where you work, or better yet, your school
A couch - Trust me, this is awesome.
A table, stolen from a Starbucks - One of the outside tables. It'll burn slow, but because you stole it from Starbucks, it'll feel good.
These are all things my friends and I have burned at bonfires over the years. Good times all around.
Happy Birthday.