Depends which Island(s) you are going to be on. January is a great time for whale watching and surfing (or watching people surf). I grew up on Oahu and spend a lot of time on Maui, so I could give you detailed descriptions of these islands (what to do, where to eat, etc...). It's been about 6 years since I've been to Kauai and the Big Island, so there may be someone here that could give you better up-to-date descriptions of those islands.
As for island hopping, I've really never done it but know friends/ family that have and they all wish that they could have stayed on each island longer.