Frankfurt has a lot to offer, hauptwache is the main street spot, then theres also bocki banks, you will probably meet some locals there.
they have a pretty mellow and fun skatepark below a bridge, its called skatepark höchst
you will find a lot of good turkish food (more than just döner) in frankfurt too, but theres pretty much everything avalaible, its a big city
in heidelberg i only know the diy skatepark, that one is cool but hard to skate.
stuttgart has a big skate scene but i havent been there recently, im not sure where to skate street wise, sorry.
maybe check out würzburg, its a beatiful city with a castle and there is a super good skatepark right by the river with wineyards in sight, also dm me if you go there
