I used that space for my battery gauges, a couple switched USB outlets, and a clearance diagram. Looking at it takes my eyes further off the road than I'd want for engine gauges, but I rarely need to look at the battery status while driving, so it works out well. I put a 2m/70cm ham transceiver up there too.
Run any wiring before you screw the thing in place, it's a real pain to reach in there after the fact. In fact even if you weren't putting gauges there I'd tuck a house power wire up there anyway, just in case you want it later.
EDIT: If your stereo has a bluetooth hands-free system, that shelf is also a great spot to mount the mic. Puts it nice and close to your head without being in the way.
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.