Okay- so this is a budget board...
Is it essential you spend it on something ELSE?
Or can you earmark something you already planned and say the gift of $500 covered that. Anything at Disney is something fun; none of it is practical, even if you say it went to the hotel room. And you get to keep your own $500...
Since you say this was a super budget trip, you are now $500 on your way to your next trip...
I like this comment / thought.
Not to be a scrooge or anything, but the parks by themselves for us are really fun and special. So is just being in Orlando. The special experiences we've done there IMHO were overpriced and not worth the money to me. lol // I'd probably use it for a few counter service meals in the park (not need to go back and cook at the RV is you're not in the mood to do that) and save the rest.
Just my opinion: People love Cirque du Soleil and if you are interested in seeing one of their shows the one in Orlando is similar and better priced than most of them in Las Vegas. It's less dark than the Vegas one. Cirque, though, is just not my thing.
Special Disney cruises, desert parties, dinner packages, to me just aren't worth it either. Anything like this I've done, I've just done once and wasn't really all that impressed.
If your kids would like to do horseback riding, the little power boats, etc. from Fort Wilderness, I think that would be a reasonable splurge too as another poster indicated.
It's your birthday, though, and we are all different and all think different things are cool and fun to do. And it's also your opportunity to do something frivolous and fun. So have a great trip and do whatever you want to do.