Swimming with dolphins. Did this in NZ
The first thing I thought of was kayaking with dolphins – also in NZ, but more specifically in Milford Sound – the whole experience was quite magical.
The next one which comes to mind is Carnac. Not sure if it’s quite so good if you’re in a car as you get a bit isolated from it, but we’ve been there a couple of times on a bike tour, so you get to ride along next to the stones and stop where you want. On a similar, but less mainstream note, riding in SW France near Carcassonne and coming round a corner to see an ancient walled citadel perched on a hilltop (Carcassonne itself is good, but not as good as that).
Top of the list though has to be XC skiing in Yellowstone National Park (far, far better than joining the crowds in summer – sorry!) Old Faithful is definitely wow, but getting away from the crowds to less visited parts and seeing hot springs bubbling up through the snow is even better. Not to mention sitting down to eat your lunch on a bit of ground cleared of snow by the thermal activity and having to get up because your bottom is getting too hot!
…actually that reminds me of a last one, Hot Water Beach on the Coromandel Peninsula in NZ. Dig your own hot pool. Probably made better because we went in August when the beach was virtually deserted and you wouldn’t normally be stripping off to sit in the water.