We are also planning a graduation party for DD in June -this is what we have planned so far ( DD has picked everything out so far)
our house is small, and the party is going to be an "open house" type- (we could expect over 150 people) so we are going to hold it at a city park, it has a enclosed shelter, a softball diamond, B-ball court, volleyball court - ( this is a family thing, when we have big summer parties, we get teams together and have fun playing)
the shelter is an added bonus if it's raining/ too hot/too cold. ( it even has a fireplace if it's really cold

)
For the cake, DD has an advantage - I am a cake decorator, and she picked out this cake -
http://farm4.static.flickr.com/3349/3592258518_3b0236d6b6.jpg with her school colors or red, white & black. Around her cake she wants cupcakes decorated w/ roses & mini motor boards like this-
http://farm4.static.flickr.com/3326/3604971688_ae3e7ca861.jpg
and homemade bars, mini cheesecakes, etc. - kind of like a dessert buffet.
For the food-( I cheated and it's being catered from our favorite Mexican grocery

) we are doing a Mexican fiesta buffet, chicken & beef fajitas w/ yellow rice, refired beans, pico, etc.
we are also going to have a few subs from
walmart for guests who do not like the spicey food. With chips, & dip, etc.
decor is just simple balloon bouqets on the tables, and she wants a power point running the background with photos from when she was little until no - like a photo montage

.
ETA: if you need balloons, i found a great wholesaler that sells deflated mylar balloons for a really awesome price, and quick shipping-let me know and i will post a link.