The kids and I have been enjoying this hot weather at the outdoor pool at Marienhöhe. It's a great place to go to have a dip and cool off or soak up some sun. It has a large pool with slides and diving boards as well as a smaller pool for the toddlers, a sauna, change rooms, and a place to get food and drink. Large grassy areas, trees for shade, and a place to kick around a ball. It's located at Lutzerneweg 1, 22589. It's about 10 minute walk from Sulldorf stop on the S1.
It doesn't cost much to get in and it is open every day 10am - 6pm, some days open till 8pm.
There are others as well in other parts of the city, but I have only been to this one because it is in my neighbourhood.
The website for locations of other pools: Bäderland