Deimos + Phobos + various bodies from the asteroid belt. Some of them, such as Ceres, could instead be smacked into Mars to increase surface heat and water presence. Velocities involved would be enormous due to the planet's gravitational pull. Care would be needed to decelerate these bodies. Even a relatively small delta V between bodies translates into an enormous amount of energy (i.e. heat and ejected mass). Collisions would vaporize a fair amount of mass, which would then precipitate into a dust cloud. Over time, the dust would either settle onto Mars, or onto the new moon.
Collisions with smaller bodies seem pretty ideal. The mass hitting the nascent moon would have its force dispersed into heat more than blowing bits off. With a regular enough stream of mass, the heat generated would melt the entire mass into a ball of slag.
It's a neat idea which could work out well in the long run, but it's also enormously costly compared to the artificial magnetosphere idea (which also comes with a planetary energy grid).
Russia landed an explorer on Venus's surface. It died quickly after broadcasting a brief amount of image and technical data. The 870 degree Fahrenheit surface temperature simply does not allow for any modern mechanism to work.No, the plan would be to have balloons floating in 1-atm, rather than in orbit. I believe that at that altitude, gravity would still be pretty Earth-like, and the amount of sulfuric acid rain would be manageable. What the people in said balloon cities would do to pass the time, though, that's where "having robots that can go down to the surface to mine" would come in.
Those balloons would need a lot of redundancy. Buoyancy seems a lot less reliable than inertia for holding up an outpost.
https://en.wikipedia.org/wiki/Atmosphere_of_Venus
50kph winds and sulfuric acid clouds don't seem insurmountable. 1atm pressure, Earth-similar temperature, and Earth-similar gravity all look fairly favorable.
I'm not convinced that Venus could be terraformed over the long run, but it absolutely has potential for the site of a manned research station. We'd need a heck of a good reason for humans to be there, though.