Triggering crystallisation of a planet's core is left as an exercise for the reader, and would be incredibly difficult, but it's a lot more plausible than trying to supply enough heat to start convection by any other means.
interesting questions might be
How much nuclear waste would have to be dumped down a borehole on Mars to remelt the planetary core; ( I know it's an insane amount, but how insane)?
- How long would it take to melt?
- How deep would the borehole have to be, at some point the waste would melt and go into "China Syndrome mode" and melt it's way down?
- Should we crash some icy asteroids into the planet to get some potential oxygen from water before or after we restart the core?
- How many rocky/metallic asteroids should we crash into Mars to get the gravity up?
- Would it just be easier to build a ring-world?
- Could we harvest gasses for atmosphere from Jupiter?