Blue Lagoon is a bit of a tourist trap but was still cool. There are prettier hot springs though, I believe I went to the secret lagoon which is on the golden circle which i'm certain you will do. I recommend you go to the penis museum, the only one of its kind in the world. Make sure to try Puffin, its a bit gross but its what people survived on in Iceland for thousands of years. Iceland is crazy expensive, it makes Vail Resorts seem cheap. The one night I went out, the 6 drinks i ordered came out to over $100. BUY YOUR BEER AT THE AIRPORT.
Finally the number 1 thing you have to do is buy a hot dog. It sounds weird but its actually really popular and really fucking good. I want to go back just to eat one of those hot dogs again. If theres one thing I would say you have to do, it would be this. Have fun!! if you have anymore questions feel free to ask.