The Blue Lagoon is a really unique experience but there are other beautiful geothermal spa's around Iceland. It's not cheap and can be very touristy depending on the time of the year you go.
BUT if you do go make sure to get the 3 types of face mud masks to wear while you are in the pool!
I used to live in Iceland and loved coming here. Absolutely gorgeous and such an important part of Iceland's history. The walk through the Tectonic plates in incredible, you can even scuba dive there is you want! 1/2 from Reykjavik and worth the drive.
Continue on to Laugarvatn, a small village 1/2 north with an amazing geothermal spa then head to Gulfoss!
I grew up on Lake McDonald, have lived all over the world and still GNP is the most beautiful place I have ever seen. Fall is gorgeous and you will miss the crowds.
Eddie's has good ice cream in the summer (closed in the winter).
Must do Going to the Sun Road! Swim in Lake McDonald! Shop at the Cedar Tree! Bring warm clothes and rain jackets/boots. Also read about bears and be prepared, bring pepper spray and stay aware.
A beautiful piece of art work. Day trip for sure takes about 2 1/2 - 3 hours one way from Salt Lake. Lots of beautiful lava rocks on the way their, a large Missile building sight in the middle of no where and little towns to drive through. Bring a lunch/snacks and water. And hiking shoes. Depending on the time of the year it might be more underwater then usual but since Utah has been having such bad winters the water doesn't come very high to it any more.
Love this place. It's a super hippy, laid back, casual spot. My friend and I rented an 1800's pioneer cabin for $100 bucks for the night, had electricity, nice furniture, wood stove, we were warm all night. The hot springs are crowded during the day, not too bad at night and empty early in the morning. Get up early to watch the sun rise and get yourself a tub, the tubs are the best part. Some are hotter then others.
If you are looking for something else to do in the area Freemont State Indian Park is 20 minutes away, $6 dollar entry, totally worth the stop.
Walk around to see some beautiful interior design/building/art. The Choir is world famous and lovely to hear.
BEAUTIFUL BEAUTIFUL PLACE. Make sure to bring plenty of water, be prepared its the desert. wear sunscreen and good hiking shoes. Find a camping spot on the river its a fun way to end the day while making dinner.