Take the boat trip from Bystrc to the other end of Brno Lake and back. It's takes a couple of hours and if the weather is sunny you can get a decent tan in one day!
It's very relaxing!
(In the pic you can just see the boat coming around at the top right.... I switched to video mode after this! The pic was taken from Veveøi Castle)
The lake is a haven of peacefulness only a short distance from the city and can be reached by tram and bus and also by personal transport (bike/car/motorcycle).
As well as sunbathing there are opportunities to swim and take a boat trip.
If you are into naturism there is (was?) a naturist bathing spot on the Rozdrojovice side of the lake.... opposite Obora I think.
Brno reservoir also Brno lake also Brno dam is the city's recreational area. Built in the late 1930's the dam of the Svratka river created this popular reservoir. With the pleasant grass and woodland surrounding it the place is popular throughout summer. Sailing, boating and swimming in the water, with volleyball, cycling, walking, sunbathing etc.. on the shore. In winter the water freezes and once safe, ice-skating and ice-hockey is taken up or if like me you just want to walk (slip and slide) on the ice, you can do that too.