17th Birthday Party

Hey, its my 17th birthday really soon and im thinking that i want to have a huge party like 50-75 kids but the problem is the weather. I have a perfect backyard but i dont know if i could trust it not to rain. The other thing is that since my parents were gonna be there no cshady business could go on, but i really dont want it to be lame. How can i make it fun and cool even if there isnt alcohol? any ideas on activities or locations. Or even if u had a different idea for a 17 year old girls party. Paintballing anything! i just need some help!