Well, there's a decent amount of stuff that isn't really solo viable for most reasonable characters but could be done in under 2 hours -- a decent chunk of the areas explore really.

That being said, keeping with the 'mostly solo' thing, I'd say that a paladin was always about the best thing for this for me. I've also had decent solo explore success with (good) conjurers, shamans, and bards. Each of those struggled with some kinds of areas that they others didn't.

Your style may be different; that's just what's worked for me.

Something else to keep in mind is that if you want to explore the underwater areas, you may want to avoid a hydrophobic race (and something like shifter, which barring a lucky draw isn't going to be well set up for both land and sea exploration).