Failing to borrow a gazebo to continue with outside activities (and believe me, that doesn't work because some children think they will melt in the rain!), this is what I did:
bedroom 1: Build a Den (lots of spare sheets, towels, wool, net curtains etc) - treats served when the den is erected and all children safely inside.
bedroom 2: A film (age appropriate) with icecream and popcorn for half time
bedroom 3 (or dining room): card games with little sweet treats. Snap, Newmarket, Follow the Ace are good for this.
kitchen: baking cakes for the party, putting snacks in dishes etc
lounge: blowing up balloons, making a birthday banner, making cards for birthday child/colouring.
At least two adults supervising per room whilst you finish off the party food. (Do a swap-over rota so adults can join in the adult time)