Enormous man-made lake with many, many attractions.
There is a sandy beach with a safe roped off swimming area, nearby pedalo hire is available.
In another area kayaks and sailing boats can be hired and lessons booked if required, or maybe try water ski-ing?
Take a riverboat tour of the lake if you prefer a more sedate excursion, or go out on a motor or rowing boat with your fishing rod.
Alternatively stay on dry land and walk, cycle, ride or drive around part or all of the 35km perimeter.
Maybe you just want to sit and look - there are lots of picnic areas with tables as well as a formal retaurant, bars and creperies to stop for a meal, or just a drink and a snack.