If you are up for a 45-60 minute hike, you can skip the blue lagoon and go to Reykjadalur Hot Springs… hike is at the end of the road in Hveragerdi.
If you go toward Vik, which most people do, you will go by Seljalandsfoss (waterfall). Definitely get out and go see it, and before you leave, you can walk to the left about 1km on a paved path and you get to Gljufrabui, which is hidden in a cave. Your feet might get wet getting in to see it, but it is 1000% worth it.
I went to Iceland for a long weekend in September and I am trying to figure out how to get back for a longer trip ASAP. It was awesome.