I had to do a presentation about Mars, and found it to be a fascinating subject. If Mars had an atmosphere that wasn't corrosive, but not necessarily breathable, there could be some possibilities. With no shield from the Sun, it is like you said, a science experiment at best.
This solar system is, aside from Earth, an inhospitable dry hole. Venus could have had possibilities, temperature extremes aside, if the atmosphere wasn't so violent and toxic.
I think out around the gas giants of Saturn and Jupiter, amoung their moons, we may find some small niches where some fragments of life managed to take hold; but no “creatures” and no civilizations.