Big ass lasers are an ideal I stole from Larry Niven (if you are going to steal an idea steal from the best). I used it years back when I ran Across the Bright Face. The PCs patron had the control codes for the launch lasers that could be used with orbital mirrors to slag the planets surface - including revolting cities. The patron died leaving the PCs with the, useless to them, codes and a lot of angry rebels looking for them. Memorable bits include the dust pools and one revolting domed city who drove a herd of horses, playthings of the spoiled rich, out of a cargo airlock into the vacuum on the surface of the planet to die horribly. One of my players with a blacksmith who hated horses [1] so he liked that bit.
If the mas driver was a bit closer to the main world, say on a moon, it could be taken by terrorists who threaten to bombard the surface with it. That sounds awfully familiar though, I think someone has already done it.
When I ran Serenity [2] the second scenario had them encounter a hick world and a farmers daughter with an interest in astronomy. She wanted the PCs to check out her observations, a planetoid was going to hit the colony. It turned out it was an evil corporate planet to wipe out the colonists and allow a fresh colony to exploit mineral wealth. The planetoid was steered by thrusters rigged around it. Not a lot of thrust, just enough to nudge it into the right direction. The PCs were able to shoot it out with the hostiles whom they discovered when their own efforts to use their ships thrusters to move the planetoid were countered - which was a lovely 'huh?' moment. It did leave them out of fuel on a rock heading past the planet but they savd the colony though they were rescued by an Alliance cruiser that came to investigate a destroyed ansible.
Notes
1. Pissed on, stood on, kicked, bitten... Who could blame him?
2. Did I ever mention what a dog of a system it has?