There is plenty to see and do on a tour to Iceland:
South Coast Tour: It's a full day, but it's a full day of waterfalls, black sand beaches, volcanoes, and glaciers. You might even see some puffins. The tour is not as popular as the shorter Golden Circle tour, but for my money, it's much better.
Golden Circle Tour: Go outside of Reykjavik and see some hot geysers, great waterfalls, and stunning geological fissures. It's a popular tour almost everyone goes on.
Tour the Harpa: This a a green ice-cube like performance hall on the waterfront in Reykjavik. Skip the IMAX but take the tour of the building.
Northern Lights: Appearing from November to the beginning of March, Mother Nature's spectacular light show is probably Iceland's most famous feature.
Play Golf: There's over 60 golf courses for a population of 300,000. Midnight tee times are available at the famous Arctic Open.