Royal Decameron in Panama as of a couple weeks ago had a huge waterslide almost completely built. Not sure when it will be completed, or if it is now. Through Nolitours.
Also the Sheraton Bijao and Playa Blanca RIU in Panama have waterslides. Through Sunwing.
Not sure about what activities are available at these resorts. (The only resort I am quite familiar with is Royal Decameron.)
Weather is usually hot in Panama at this time of year.
Prices, well, that can be hit and miss. Last minute to Royal Decameron can be nicely priced. Not sure about the availability though.