It doesn't get more touristy than the Blue Lagoon in Reykjavik. On your way to/from the airport hordes if tourists cram buses and descend on this natural geo-thermal pool outside of town.
You want to know why? Cause it's awesome. I don't know if it's a fountain of youth or not but it's warm, it's blue, they serve cocktails and all the algae mask you can slather on. For someone who prefers to travel off the beaten path, this is a path I'll be beating down again.
Tip: get the deluxe pkg which includes lunch (but not the private room). It's a good deal and good food. But buy your product on the plane, it's cheaper for some reason.