The Giant's Causeway 

A stroll down to see "the rocks" at The Giant's Causeway.
It's free if you just want to walk down to see the Causeway, and enjoy a walk along the coast. Charges apply for car-parking, and the National Trust bus which will save you the walk down. It's about a mile down from the National Trust visitor centre, and quite steep coming back.
We were slightly underwhelmed - but it's a must-see if you're in this part of the world.