How do I care for the land area directly above a septic tank?
You can pretty much do anything above a septic tank, as long as you aren't going to be doing major landscaping that requires tractors, heavy equipment, etc..

The one thing you do want to be aware of is the location of any inspection pipes, or manholes that might have a negative effect on mower blades.  Trying to mow around these nice yard "features" can be quite frustrating.  For our old system, we mulched around the area containing the manholes and inspection pipes (~ 4" white pvc pipe with a cap), and planted a nice flower garden around it.  We had our 3-year pumpings scheduled in the early spring before we plant, and if we needed work done on the system, flowers are easy enough to re-plant.  There are also some realistic looking fake hollow rocks you can buy- we'll probably do this at our new place, as the covers are in an area not well suited to a flower garden.