Want a truly unique experience? Check out inside the volcano if you're in Iceland. You literally go into the magma chamber of Thrihnukagigur, a dormant volcano. How cool is that?! It's the only place in the world where you can do this, which qualifies as pretty darn cool in my book.
After trekking for 2 miles across a lava field, you make your way to the rim of the volcano where you see this crane that lowers an opened elevator down into the volcano. It's actually the same kind of contraption window washers use along the side of skyscrapers. The whole thing seems like they went out of their way to over-insure your safety.
You get strapped to the walkway as you make your way to the elevator. There, they clip you in to the elevator and then unclip you from the walkway. All solid safety procedures. The biggest risk is your iPhone slipping out of your hands as you try to take pictures while on the elevator. The slow ride down shrouds you in a bit of darkness until you get to the bottom of the artificially illuminated magma chamber.
Overall it's not the cheapest tour activity out there, but I think the uniqueness of it gives you a good bang for your buck.