Sorry if I am slightly off topic or if this has been covered before (it´s a long thread). I am thinking about using the Comrades Finishers Programme from the race´s official site for another ultra I´m gonna do in the end of july.
As the last run in the fourth week in april the programme says "Rest or 50-55 km long run". Is this a typing error? Sounds a bit weird that you should either rest completely or run 50-55 km (app. 31-34 miles). Maybe it shall be 50-55 minutes?
I did the Laugavegur ultramarathon last summer, which is set up by the same organizers as the marathon. I recommend you to do the Golden Circle Tour and a Whale Safari from the harbour. The Blue Lagoon is said to be overpriced and overhyped so I didn´t visit it. There are however a lot of baths in Reykjavik with hot pots that I recommend you to visit. As they are run by the City of Reykjavik they are comparably cheap, about 4-5 pounds if I remember correctly.