Antarctica. It is the only continent I have not been on, and I would very much like to go there. I did have the opportunity at one point; unfortunately the fortunes of war sent me to the Middle East instead.
One of the objectives of going to Antarctica would to do several ice dives. While in graduate school, I studied the glycoproteins that keep several species of fish from freezing in the subzero temperatures of Antarctica. I often wondered what that world would be like, and what an experience it would to be to visit an environment that hostile. What do the life forms that live there look like? I would love to see, and live that.
I do think I would like to do a winter over, just to see what it would be like to survive there. After that, I think a long visit to the Caribbean would be in order.