What a wonderful itinerary!
Our British Isles cruise was in July 2009 on HAL. Of your ports, we visited St. Peter Port, Dublin and Belfast.
In St. Peter Port, Guernsey, we took a ship's tour -- a general island tour. We drove along the coast for most of the way, and made a few stops, some kind of touristy, some very interesting. I suggest reading up on its history before you go, if you haven't already, most notably the occupation during WWII. There is also a local bus that does a circle around the island, although you'd have no commentary. But, knowing the history and geography beforehand would be helpful if you opt to see Guernsey on your own.
We took a ship's tour in Belfast that was amazing, and geared toward the nature lovers. Do a search for Giant's Causeway. It's an unusual, and very large, rock formation located roughly 70 miles northwest of Belfast. It consists of more than 40,000 huge rocks projecting into the ocean. I cannot, with a description, do it justice. The views were glorious, and the sights were interesting and breathtaking. As it was a long tour, we also made a few photo stops, and had a wonderful lunch at a local restaurant on one of the cliffs overlooking the ocean. I highly recommend it.
We did our own thing in Dublin. There is a wonderful and very convenient hop-on-hop-off bus that covered a lot of ground. This might sound strange to some, but the highlight of our day was visiting the Dublin Zoo, which is on the route. We enjoy zoos, and this one did not disappoint. Once back in the center of town, we did a bit more walking, including a visit to the Trinity College campus.
What are some of the tours you're considering? While we didn't call on your other ports, we might have been nearby.