The excellent Reykjavík Art Museum is split over three well-done sites: the large, modern downtown Hafnarhús focusing on contemporary art; Kjarvalsstaðir , in a park just east of Snorrabraut, and displaying rotating exhibits of modern art; and Ásmundarsafn , a peaceful haven near Laugardalur for viewing sculptures by Ásmundur Sveinsson.
One ticket is good at all three sites, and if you buy after 3pm you get a 50% discount should you want a ticket the next day.