I agree with renting a boat and going tubing, or jet skiing on Table Rock Lake. We stayed at Big Cedar and they have a marina there, but I think the State Park marina was cheaper.
We did almost all inside activities while there, it was 100+ the week we were there. One thing I will suggest, is pick all day, one destination things to do on the weekend so you don't have to drive around Branson during the weekend. The traffic is crazy and it takes you a long time to get from one side of town to the other. Weekday traffic was much better.
My kids are younger, but we did the Titanic Museum, Butterfly House, the Ducks, a Chinese Acrobat Show, Dixie Stampede and we walked around Branson Landing.