I think a warm, wet Venus is mandatory, but there's no reason it all has to be jungle. Monstrous crab-things in the boiling equatorial seas are fine by me, as are wasp-man hive-villages in the high mountain peaks and all sorts of reptillian types on the polar continents. No matter how you slice it, not a great place to visit, and you'd hate to live there.
Re: Martian invasion status, does anyone remember the old Devil Dinosaur comic? One storyline featured high-tech alien invaders who come down, start slaughtering dinos and cavemen left and right, then get swarmed under by giant army ants. Silly, but to the point. Neolithic tech does not equate to harmless, especially when local animal husbandry skills involve dinosaurs and the like...
Rich