Frankfurt:
- Ghetto Spot (there's only one half (yet, the better) left and soon the whole area will be knocked over... still, the better half could be still skateable during your visit - no guarantee, though)
- Park under the bridge (good park, a little too dark)
- Hauptwache (legendary street spot but haven't been there for a long time)
I guess these are good or the main starting points... For further spots, talk to the locals