If you're looking for ways to keep the kids and adults entertained so they stay at your place instead of going somewhere else, you should seriously consider putting in a waterslide (or two). The right waterslide can help bridge the play gap between adults and kids and will complement your other water activities.
I love facilitating active play with kids, adults, and everyone in between. Here's a few strategies I've used to increase the level of fun (and exercise) for our family during summer break. You don't even need a waterslide (although they're really good at setting the stage for active play).
I got my start recycling water park slides after seeing one torn down and trashed 8 years ago. For me, the best part of slide recycling is that everyone wins: parks win (easy way to get rid of old slides), you win (how about a 150 foot slide going into the lake from your back door?), and I win (we make a difference one slide section at a time).