So far, I've always moved all my automines to somewhere else, and move some mines to colonies so they are busy.
Other things they can do is run maintenance facilities for forward fleet bases, and replicate.
Send them a few dozen factories, and they can build more.
If you have the required minerals (important), you can also build a lot of financial centers for higher tax income.
And don't forget to get atleast one level of civilian spaceport on every planet, that boosts trade and creates more income aswell.
If you don't have a reasonable civilian shipping line, you can subside them to boost that economy.
Once you produce enough wealth, research is a good way to burn work power without burning resources aswell.