I work weekends all the time, I don't get paid extra...but I'm salaried, etc etc...
You're not. You're an hourly employee. If I were in your shoes? Absolutely I'd expect to get paid. I would bring it up to your boss. Sounds like you're a pretty level headed person, so it's not like you're going to be storming in there demanding anything. Simply stating, "You know [insert boss' name here], I was wondering about this fair coming up. I was asked to run the bean bag toss. I've been trying to figure out if it's something I'd get paid for, could you clear that up for me?" If (s)he says yes, then you can say, "Alright, I just need to make some arrangements at home." If (s)he says no, then you can say, "Thanks for clearing that up. I'm not sure how that weekend is going to shake out yet, I'll have to see if I can make arrangements." Then you've bought yourself some time to back out.