I've done it plenty of times in yachts, haven't yet taken the RIB over though - is on my "to do" list.
Herm (Shell Bay or beside the half tide harbour are my favourites) will be an easy and short trip in a RIB from Guernsey, and makes a lovely day out. Not that much ashore, but a great, tranquil island to walk around, or enjoy the many beaches.
Sark is probably my favourite Channel Island, always something going on ashore, and you can have a wonderful cycle round the island in a day, stopping for lunch somewhere en-route at one of the many local cafes/restaurants. We normally anchor in Dixcart Bay or La Grande Greve, or pick up a mooring in Le Havre Gosselin. I know some RIB's get permission to stay/dry out in Creux Harbour.
Alderney is only ~20NM away, so not far on a RIB on a good day - plenty of videos on here of people visiting Alderney by RIB.
Definitely make sure you have tide tables and suitable charts for where you're going - plenty of things to look out for around there and large tidal ranges, but the main routes between the island are pretty well marked.
Quite a good guide to the closest anchorages here: Anchorages - Guernsey Harbours