The more complete answer would be to bioengineer creaturse to live on Mars. Possibly including human-life analogues.
Using human DNA as a starting point (instead of developing the blueprint for intelligence all on our own...), we'd modify it with thicker integument against the radiation and cold, possibly some form of anti-freeze in the blood, much bigger lungs and less need for oxygen.
Unfortunately, intelligence needs a lot of O2, so you can't cut back on that part, but at least metabolic and muscular functions can be scaled back.
That kind of life can live on Mars within a few thousand years of successful plant-life seeding. But it would take a longer span than that to make larger patches of the planet arable.