We were there 2 weeks ago and had a full day on arrival from the US, then a morning before we left in Reykjavik. I think it depends how much you want to do. We didn't look into the card ahead of time, but we spent a lot of time walking around. The one museum we went to, the Settlement Center, was free for senior citizens, so if you fall into that category, you can see if that applies to other museums. They did ask for proof of age. The weather was never very clear to pay to go up the Hallgrimskirkja Church tower.
We ate dinner on the harbor both nights, Kopar and Hofnin, and reservations were a necessity. On arrival, we had breakfast at Sandholt Bakery, in the RS book, but recommended by our hotel, also. It was worth the 10 minute wait. We had lunch at a small place called Supa, which serves delicious soup and bread. It was about $14 each (a good price in Iceland!) and was off Laugavegur, the main shopping street. Enjoy your trip, and dress in layers!