When it is summer, you can go to the leisure pool Heveney in Dortmund at the "Kemnader See".
I like it personally very much because, you can swim, slide, splash, dive, have a sauna and chill. It is a fantastic place with lots of alternatives to have fun.
There is a children bathing area and a whirlpool, two slides, one of which is a tire slide and a 100m long outside slide.
A cafe restaurant is also available to guests.
The first best thing you can do there is to try the 100m slide.
The second best thing you can do there is to relax in the whirlpool.
The third best thing you can do there is to buy ice cream at the cafe.
Also you can go for a walk on Kemnader lake after swimming.