I'm working with a complex shape, but for the sake of getting a solution, let's say I draw a circle with the circle tool, and then I do a smaller circle inside of it. I select both objects and use pathfinder to erase the smaller circle on the top. Now you should get a "donut" shape.
How can I fill the inside of the donut? Is there any tool to create a selectable shape out of a hollow shape?
You have what is called a compound path.
You can Right Click → Release Compound Path and then fill each individual shape.
This works better on simple shapes, as it can have some unwanted side effects. For more complex shapes, see the next solution.
You can use the Shape Builder Tool (Shift+M) and then just select your path and click in the empty area you want to convert to a shape.
Ctrl click will add a path
Alt click will remove a path
