V.B.
Considering where you live, I don't see any problem with stating two options. I think it is a great idea. One idea for a way to word the invite... give all the information like it is going to be held outside, swimming etc. Then at the bottom of the invite put something like "in the case of rainy or inclimate weather, we will hold the party at ..." I've seen invites like that and not thought it odd at all.