Mercury: we only know a fraction of its surface and its a lot closer than Pluto.
Also, being much closer to the Sun, there certainly are interesting properties related to this, such as much much more He3 than on the Moon.

Venus: a technological challenge, a greenhouse planet gone bad and the closest planet.

Titan: the famous atmosphere rich moon of Saturn!

Uranus has a moon, Miranda with a 20 km high steep cliff!

Neptune has a moon, Triton, that has gases escaping from it, in spite of the cold.

What the great voyages of planetary discoveries have taught us was that every world is different from the others, has its own history, evolution.
It's just that some worlds are more fashionable than others.

Raoul