I would guess that the majority of visitors to Reykjavik by cruise ships stay in the capital and, indeed, it is a lovely city, albeit with a lot of development currently taking place.
However, get out into the surrounding countryside and you will be blown away by the stunning landscape - vast lava fields covered in bright green mosses and lichens, mountains, roads edged with wild lupins, deep blue-green lakes and steaming volcanic mud pools.
I would suggest a half-day tour and then the rest of the day in Reykjavik.