"What are you doing with that?"
Translation:Wat doe je daarmee?
It's simply not used, probably because we have daarmee. :) Similar sentences like Wat doe je met mij? (What are you doing with me?) or Wat doe je met dat voorwerp? (What are you doing with that object?) are used.
Daarop can also be translated as on that, though it may depend on the context and the word order might have to be considered as it does not always work to translate it word for word.