We live in the country and chose to not have any AC nor heat put in our house (we heat with a wood burning stove), and there are a few "warm" days, but nothing bad and it always cools down into the 60's at night. But on those days you don't want to cook either because of the heat or just don't feel like cooking; here is what we do.
In the summer I always have a huge bowl of potato salad, and coleslaw in the refrigerator and bowls of cut up melons, and any other fresh fruit that is on sale. Bags of cleaned and cut up veggies make a quick sandwich go with or to use with a dip for a snack. I alternate between tuna salad, chicken salad, cold cuts, cooked and refrigerated salmon patties( we reheat in the microwave). Then for veggies we usually pick what is ready in the garden ( or what you can get at the farmers market) and steam those (steaming is a short process so it doesn't heat the kitchen). You could precook what your family likes and also just heat per plate in the microwave and that doesn't heat up your kitchen. If we want baked goodies I usually bake at night; but since you are in Germany lucky you can go to the bakery and get all kinds of goodies!
We also do smoothies a lot for lunches and sometimes even breakfast. Homemade breakfast bars are good and they require no cooking or next to none.
Puddings and jello make good summertime desserts with a little cool whip on them.
I probably have some recipes if you would like just let me know! Enjoy Germany while you are there.