A lot of malls have indoor playgrounds... and I would find ways to play in stores anyway... like being a train and playing follow the leader thru all the aisles of a clothing department store is actually fun. What other indoor places / attractions are in your neighborhood? Is there a children's museum in your town - often they will have membership deals for the summer. When bored with their toys mine discovered the pots, pans, and gadgets of the kitchen were fascinating. Blocks are always fun. Get out the sheets and make a fort inside the house. We even put our camping tent together in the house and went camping in the living room.
A small pool that you could put away might work, if not, just playing with the hose used to entertain mine pretty well.