Earth is pretty easy to do if you do it the simplest way, via atmospheric braking all the way down. You have to have a heat shield and parachutes, but if you ha

Earth is pretty easy to do if you do it the simplest way, via atmospheric braking all the way down. You have to have a heat shield and parachutes, but if you have the equipment the atmospheric braking doesn't require any fancy calculations besides getting the entrance angle right (so you don't bounce off into space or burn up by going too steep). Mars is probably the worst of both worlds, you have a thin atmosphere that is a nuisance but not very good for braking.