Nothing comes to mind immediately, but Google: "august 20, 2005", florida, resort (use the quotation marks around the date) to get a ton of ideas. Pick something close to you to cut transportation costs and allow you to do a little recon this weekend. Sure don't want to throw a party in a dump. (OK you are going to get hits on several reunions (have you seen "Wedding Crashers"? . . . no, bad idea . . ), at least one nudist resort (worse idea), but you should get some pretty good ideas out of an hour or two's surfing. Call your best venues for details and arrange a visit if possible before you set this up. As you know, you are working on a VERY short schedule - unless you choose something nearby.
You might have something like a museum or a zoo, or another private venue that is available "after hours" for a unique theme party. I often work with a warbird museum in Galveston that is a real indoor, all-weather showplace - spotlessly clean, polished floors, beautifully restored (and flyable) aircraft on display - that is often used for banquets and *****, corporate parties and private events of all kinds. Appeals to gearheads of all kinds and the ladies love the brightly painted and polished aircraft (all historically significant) and the bright, colorful facility.
An arboretum or nature trail could be the scene of a unique picnic-themed event, or a natural history museum, or a private zoo, the setting for a costume party (don’t come as an edible
species). A mini-cruise aboard a charter yacht, a weekend at a golf or tennis resort, maybe a hot-air balloon tour – all depending on your own interests, your guest list, if kids will be present (petting zoo?), budget, and what’s available. You could combine this event with a weekend getaway at a local resort for you and your friends. If you find a resort, ask the staff for ideas they can produce, or what’s available in the area.