Swimming with huge manta rays at night. That was in Hawaii, when I was there once to go to work at one of the telescopes there (there are quite a few telescopes in Hawaii, on the top of an extinct volcano!). Manta rays are _huge_, but they’re beautiful. It was really exciting to snorkel with them, but quite scary to be in the middle of the sea at night. An amazing experience!
The problem is that what I find exciting you might find as dull as dishwater! 🙂 For me many of the most exciting times I’ve had have been at rock concerts. The Monsters of Rock/Download festival is always pretty exciting for me but one of the most exciting events I’ve been to recently was the 50th Anniversary Dr. Who celebration in London a couple of years back.