A lot of companies rent out different parks, lands or rides. Schools rent them out for Proms and weddings happen in front of Cinderella's Castle!
I love Jungle Cruise, but I can't think of an what you'd use it to celebrate! I think Expedition Everest (and the surrounding area) would be fantastic for a party in the evening. So much fun.
But really the BEST place for me would be the Mexico pavilion, especially with La Cava del Tequila. All that great snacky/sharing food and most importantly all those margaritas! You could have a nice dinner under the fake stars. Blast whatever music you wanted around the enclosed pavilion. Go on the cute boat ride to cool down.
Or perhaps Le Cellilier? I love my cheese soup!
After a good ten minutes thinking time, mine would have to be Toy Story's Midway Mania in Hollywood Studios... I need time to hog the controls until I get better at all the games.
Of course I'd be having a party too, so all my friends and family could practice the games, but we'd have food and drink served in the queuing area. We could climb the giant toys and hold races... (ahh, I was back there then...)
I'd have to say my personal choice would have to be Imagination. Not only to explore the attraction but I'd find the power controls and turn the playground upstairs back on then spend hours playing.
Otherwise it would be the old "Wonders of Life" pavilion so I could explore it's remains. Check out Body wars and Cranium Command (Still there) plus finally see the lounge upstairs!
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal Studios Florida
Universal's Islands of Adventure
Universal Studios Hollywood