It seems like Callisto may also have a subsurface ocean too.
At it's largest Ganymede has a diameter of 1.8 arc seconds. Under ideal conditions my 14" telescope can resolve details down to 0.3 arc sec so at best just the broad details. One group of amateurs has started using 1m (40") class telescopes. A recent image I saw from them had perhaps half the resolution of hubble telescope images. Btw here is what hubble is capable of.
Those pictures really aren't bad at all considering the scale - I can see quite a few details on all four moons.
Looking forward to your assumed next post on Callisto!
I think it MIGHT be possible for advanced amateurs to do continuous monitoring of the Satellites and perhaps monitor for activity, like water releases from Europa, Eruptions from Io, etc. You may not necessarily have resolution to see the event directly but using a spectroscope look for the relevant signatures.