Resources gathered from mining will deplete. Eventually, after enough people have scoured a planet, it will be devoid of its mineral bounty.
One way to mitigate this is to have has some resources be renewable. The simplest example is trees. Having plantations of trees ensures that there will always be a supply of wood.
When people think of farming, they think of crops and livestock. When people think of renewable power they think, solar and wind. Can we do better?
What about mineral crystal farms? These might require the soil of a specific biome and through a process that requires quite a bit of time and power (with lazors!), constructs a mineral crystal which can be harvested. This mineral could be quite scarce if not rare to mine normally.
Another renewable resource could come from collecting the atmosphere of a particular planet. This gas might be difficult to find otherwise. Creating the elements to collect the gas might be quite expensive to create and require power of their own to run.
Renewable power could be thermal energy power collected by a specific element from heat deep inside a planet. Planets that do not have a hot core, will not be able to sustain these type of machines. Maybe a coolant would be required or the thermal power element overheats and shuts down.
If these renewable resources/power are spread over different planet types, it encourages trade from those high density points to places where there is less.
Can you think of more?