Most of our time was spent outside, but we found plenty of stuff to keep us happy inside, too.
We didn't hit too many museums on our visit, but the two we did get to were awesome. First, the Lava Centre in Hvolsvöllur is a must. My kids are still talking about its interactive displays on plate tectonics. Which, if you know my kids' views on the merits of museums right now, is really saying something. Rarely have I seen so much solid information conveyed in such an engaging way. And the museum cafeteria is pretty good too.
The Settlement Centre in Borgarnes also made a big impression on the kids. Alright, and me too. The museum contains two very different exhibitions: Iceland's settlement history as documented by historical and archeological fact, which is super interesting. The second exhibit lays out Iceland's creation myth using Saga. Specifically, Egil's Saga, which has all the makings of a Hollywood tale: dysfunctional families, blood and guts, rags-to-riches, sporting events gone horribly wrong--there's something for everyone. Super bonus: the Centre's cafe has a nice lunch buffet. And lamb soup.
Another fun indoor activity is a visit to Fritheimar, a really cool tomato greenhouse. You don't have to be a gardening nerd like me to love it, although that helps. It's bright and warm inside, which you'd expect from a greenhouse. But it's also a restaurant, which you wouldn't. Lots of tomato-based dishes on the menu (duh), and as always in Iceland, try the soup. After you eat, you can wander around the greenhouse, where there are exhibits on the history of the farm and the mechanics of growing food indoors. I also loved the bees, which pollinate the tomato plants, herbs and flowers grown in this state of the art, geothermal-powered greenhouse.