We had half a day in Reykjavik, and besides visiting a thermal bath, what could we do? Obviously, visit a cave... a lava tube. Or the Lava Tunnel, few kilometres outside the city (coordinates: N 63.9402 W 21.3971).
This is a very nice, organized setting to view an easy-to-walk-through-but-interesting lava tunnel from 5200 years ago. Additional show caves can be found on this list. And there'd be a great inside-a-dormant-volcano tour also in Reykjavik, but unfortunately that cost 400€ so we decided to skip it.
I also wanted to go out and look for wild caves but we didn't have time for that this time. Apparently, there's at least one near Reykjavik that can be visited. And even the lava fields next to the airport seemed like they are full of interesting holes to peek into. Maybe next time.
The most interesting part was the sinkholes viewed from underneath, with snow falling down to the cave. And the colors and dripping lava forms. Like this: