Something able to refuel at Titan (just suck in the atmo, chill slightly to condense the methane and vent the N2) and then scoot around on a Nuclear Thermal Rocket should do the trick - it can land on and analyse just about every moon. Zubrin's original couldn't reach Mimas on one tank, but Enceladus probably has easily accessed sub-surface volatiles - heck Mimas probably does too. A heated "ice worm" and a pump should be able to sample and refuel at the same time.

Adam

PS According to the latest density figures for Enceladus it seems to be enhanced in silicates relative to the other inner moons - explaining the apparent volatile releases thanks to radioactivity and tidal stresses?

Mimas 1.162 ± 0.013
Enceladus 1.830 ± 0.170
Tethys 0.991 ± 0.008
Dione 1.489 ± 0.040
Rhea 1.230 ± 0.039

JPL Satellite Data (Nov 30, 2004)