Farming and passive income, combined with planets potentially representing an enormously powerful defense mechanism.
I'd like to see planets be able to be given orbital defense cannons, which operate sort-of like an "inverse" orbital bombardment going by the system
Captain Fortius described. Link weapons computers or turret dock points to an AI module, and set targeting parameters. The AI module detects what kind of entity it's been placed on, and if it's on a planet surface, it gives the option of targeting entities in space, and saves the details of the turrets/weapons and its targeting parameters, as well as the locations of the weapons. Entities approaching a planet that fit the parameters laid out in the AI screen are automatically fired on from their recorded position and don't stop until the region they're firing from has been suitably bombarded, even without the blocks being loaded or the damage being calculated on the actual planet. Ingame explanation would be that the planet's military staff evacuated the position of the turret.