2 layers pcb : ground plane at bottom layer, what to put on top?
You can pour Vdd on the top layer. The added capacitance will help decoupling. It's also slightly better usually to have Vdd on the top and GND on the bottom in case the bottom accidentally contacts something grounded.

If you're doing pours it's a good idea to increase the clearance for the pours to something big like 0.02" (0.5mm) so as to improve manufacturability. That may result in more areas without copper but you can stitch pours to those copper deserts with a few strategically placed traces and vias. You should set your EDA program to remove or avoid isolated copper islands, of course.