Pluto

NASA rendering of Pluto, the sun in the distance, and one of Pluto's moons -- perhaps Charon, Hydra or Nix -- in the sky.

Of all the planets, for sheer mystery I'd love most to explore Pluto. Many will think 'hugh, just a chunk of rock with ice, who on Earth would want to go to such a wasteland?' Well, the same was said of the Arctic (and still is today) by people whose experience in the world, and imaginations, are small. Open your eyes and the finest structure of ice differs in shade as composed to where it is exposed to light, or the reflective qualities of certain rock differ by time of day, latitude and a hundred other factors. That's it then - off to Pluto! I would go tomorrow, if I could.