We did the Greenland and Iceland cruise in 2022, on the Star, that went to Norway instead of Greenland. The best Iceland excursion we did was from Isafjordur to the Bird Island of Vigur. It's a half hour journey to the island on quite a fast, powerful boat. You can sit inside but for seeing the amazing scenery sit outside. Make sure you are wearing plenty of warm clothes!
The island is a haven for Arctic bird life, be prepared to be bombarded by the Arctic terns as they protect their nests. You are given a stick to hold above your head which the birds will attack instead of your head. Plenty of puffins and other birds to see as you walk round the island.
At the end of the visit you'll be given coffee and delicious homemade cake.