Get out of Reykjavik and go on a bus tour. I did a mini-bus tour, less than 6 people and was taken to a little town called Vik on the beach, black sand etc. We also went to a glacier, waterfall, geysers etc. Great times.
Take $$$$$$$$ bloody expensive.
Blue Lagoon, save for the end of the trip and go on your way to the airport, nice way to say goodbye as you relax in the spa. If the time is right go and try see the Northern Lights.
If you like horses they have plenty of horseriding tours too, even if you are complete novice.
Standard European plug adapter required.