T.H.
you could send out in the invitation that you are unsure of the location but you will call/email everyone the day before at the latest on where it is. give them the two options in the invite and if the weather is too bad then change it to something indoor. with this though you would have to pick something indoor that didnt have to be reserved. the rec centers have rooms you can rent for like 30 bux though. check into that and you can decorate it and they have tables and chairs then you just bring in your pizza and cake and suck you want for the party.