First: dive with true exotics the Giant Squid (or variants thereof), the Coelocanth, or the Greenland Shark.
Admittedly, the GS is too deep to dive; even if it wasn't after watching that documentary where they took it's picture I think it could be a VERY dangerous animal. Coelocanth is also too deep to dive, except at the apparent limits of rec/tech diving. I do know of people who've seen the Greenland Shark, however.
Second: a 20 ft and under dive on a warm reef, wearing only a light wetsuit, no current, lots of space, abundant unmolested fish and invert life. Hmmm I think I could dive in this place forever.......