Throwing teenage daughter a surprise party. Help Please!?

So here's some basic info, I want to throw my daughter a surprise birthday party.Her 17th birthday is next week. She said she wanted to have a party and I told her we'll see. She would never expect a surprise party. I would rather do it not this coming weekend, but next. So I could kind of put off things so she would be thinking she isn't having a party. And I would have more time to plan things and get more money for a party. (No really expensive ideas like a dj please) & she really wouldn't expect a party to be a week after her birthday. She really loves this camp ground that is pretty close. And she mentioned if she had a party she would want it to be there. It has a beach, so they could swim and I thought about renting a pavilion, I was thinking about having a barbeque and making her a topsy turvy cake. For decorations, I don't want them to be too "little kid" looking. Most of her friends are older than her & I wouldn't want anything that looks silly to embarr her. Any ideas for decorations? And to let all her friends know about the party I thought about just contacting them through facebook. She just graduated last month (she was home schooled) & the family is very proud and we want to surprise her. How would we get her to the party without it being obvious she's having a party, I would have to be there early to set everything up. Should I get her friends to make it seem like they are just going swimming there? What should they do for activities? Also, we had a savings account set up for after she finished school we could get her a car. We would like to give her the car as a present at her party, when would be the right time to give her the car? I don't know too much about parties, and she hasn't had a party since she was 13 so I don't know what to do for her at 17. Thanks for any ideas!!!!!