One of the most famous places that I had heard about in Iceland was the Blue Lagoon. It is a hot spring pool full of natural minerals and silica. It's milky blue color, 99 degree water and beauty made it a must see. Fears of the locker room were not a big deal. No one pays any attention to you. After being up all night flying, it was a delightful treat.
You walk in through a lava field.
A great treat in expensive Iceland, was the free photographer. He would take your picture, for free, and send it to your email. As the entry fee was steep to get in, it felt like a bargain to get something else for it.