I just saw your itinerary on the disneycruise.com page, it looks amazing! We stayed mostly in Reykjavik, but we did venture out to the Keflavik area and the golden circle as well. In Reykjavik we did an really fun 2 hour segway tour of the city, beautiful scenery and a good overview if you are only in town for a few hours. The owner Linda was our guide and she was great, took pics of us that she emailed as well. You wear a helmet with a mic on it so you can hear Linda's talking and describing everything as we went along. Sway Segway Tour is her business and they offer 3 times a day with pickup and drop off at the cruise ships as well
We also visited the Blue Lagoon in Keflavik, which was a real treat. Touristy, but a must do
All the tours in Reykjavik will pick you up and drop you off, which is great! We were limited on time (well we had time, but didnt want to spend all our time on a tour bus) so we chose the evening Golden Circle tour to see the natural sights in Iceland. Very glad we did that, they were breathtaking, but may be a little tedious for young ones...
Reykjavik is a small city so walking is an option, and a great way to spend the day if you are not into tours. Great food and people