Aerobraking
On Callisto or Gannymede?
- No atmosphere!
You might get away with a skimming pass through Jove's outer atmosphere and aerobrake into a transfer orbit to C or G, but you still need to do the final orbital insertion manouevre with real fuel.
Also, I think a practice on Mars would be smart before going all that way out, if nothing else to make sure your feet don't freeze to the ground at the prevailing temperatures there!