Ken M.
Yelp
Floating downstream, walking back up, floating downstream, walking back up, floating downstream... and going for a Cervelat and and Apfelwein. Pretty awesome way to spend a summer afternoon, if you ask me.
The swimming stretch is quick-flowing, often lifeguarded, and has several ladders; during nice days, expect to deal with people hanging out on the ladders, trying to decide if they want a dip or not. It's totally fair game to bypass them and jump in. (And if that's your style, there's a rope swing on the opposite shore).
The playground has both water and dry sections for the 2-to-8 crowd; the climbing structures are all pretty small, so more adventurous kids may be happier swimming.
The buildings in the center of the island have an outdoor restaurant, plus bathrooms and warm showers.
In the middle of the island, there's a run-of-the-river hydro plant --- good if you like crashing water. More impressive falls are on the south-east corner.
One thing to know ahead of time: The western third of the island is a de facto nude area, predominantly male and frequently gay. Everyone's pretty mellow, but it does make for a marked transition just past the powerplant.