Possibly Venus, although the atmospheric obstacles there--high heat, composition--will be more difficult to overcome than Mars. Until there is a plausible theory for interstellar travel, we humans are locked in our own local system. Since the only rock planets are the four closest to our sun, we're limited in landing options.
I suppose a mission to one of Jupiter's moons is on the table somewhere. The problem with that is how to launch a ship that's landed on a moon of Jupiter. In my opinion, Jupiter's gravity knocks the idea into the fantasy realm.