I have seen the shadow transits of Jupiter's moon regularly with my 70mm & 120mm refractors although Io is a challenge. The red-spot is very hard to impossible due to its incredibly low contrast, however the indentation within the cloud band where the red-spot is is visible. The ice cap on Mars is clearly visible in the 120mm.
I have found a light-blue filter works well, say an 80A and also have success with a light yellow one. Neutral density can also work on something bright like Jupiter.
For Jupiter, grab the programme Jupiter 2:
http://www.astrosurf.com/rondi/jupiter/ to give predictions of what the moons and red-spot are up to.