Day 2 and 3 - Reykjavik all the way
Lacking better words to describe Reykjavik from a visitor's point of view I would have to call it strange.
While the people are really helpful and motivated to help you get your money's worth e.g. when in a restaurant or a bar (and it's quite some money that you leave there), I have a hard time saying the same about about most museums and exhibitions.
While there are notable exceptions, like the National Museum of Iceland (being among the cheapest and having the most to look at) and the Volcano House (2x 30 minutes movies about volcanoes and hands-on exhibits of everything from ash to basalt rocks) the other exhibitions I visited were a bit underwhelming for the asked price.
Putting the exhibitions aside, it's always free to walk through town and have a look at the architecture. And in this category Reykjavik as one bad-ass of a church to look at Hallgrímskirkja
Fun fact: The statue of Leifur Eiríksson that stands in front of the church was there first (it was presented to the people of Iceland by the USA for the 1000th anniversary of the icelandic parliament in 1930). Construction on the church started 15 years after that.
Little bummer here was that the observation deck on top of the church tower was closed until the end of May for installing a new elevator.
Overall, Reykjavik is a nice town that can be visited within two days. The only stress you might get is getting into the state/town-run museums as they tend to have similar, short open hours (10:00 - 17:00).