Well, you could go the outside route again but make it a water theme. Have water balloons, water guns, and wading pools. Make sure you tell all your guests ahead of time so that they come in swimsuits and/or clothes they can get wet.
Alternatively, you could have the party at a place like Chuck E. Cheese, Kid Company or Giggles and Smiles. These are places (in the Pittsburgh area) where kids can play. They usually only charge for the kids attending the party and usually you get your own little party area or room depending on the venue. Also, they are air conditioned. We had my sons birthday party (3rd) at the Kid Company last year. The price was about $150 after I paid for extra food for the adults. The party price included pizza and drinks for the kids, a party room for the party, and a host/hostess from the Kid Company to help with setup, the party itself, and cleanup. Since I was 8 months pregnant at the time (July 1), it was absolutely fabulous and worth every penny. I figured out that I would have spent about that much just on hor d'oerves if I had planned the party in my home. Plus in my home I would have had to deal with the mess and cleanup both before and after the party.
I'm not sure if you are in the same area but check what is available in your area and what the cost will be. There are so many places to go for birthday parties that are reasonable. In the Pittsburgh area alone, there are the ones I've mentioned plus the Pittsburgh Toy Lending Library, the Center for Creative Play, the Children's Museum, the Carnegie Science Center, Build a Bear, Libby Lu, McDonalds and so many other places. Think about what your daughter likes to do and look around and check prices.