I paint my base plywood white, then stretch and staple polyester batting onto it. This makes a flat surface that is firm enough to stand most decorative items on it. The choice of batting is whatever JoAnne's fabrics has in stock that is thin, dense, and firm - almost like felt. You can use white felt but it's more expensive. Avoid the thick fluffy stuff for most areas near track unless you enjoy unwinding fibers from axles and running gear. Track, buildings, trees, figures, etc. go on top of the base batting layer.
For small figures that need a rigid surface to stand, I glue them to white foam board, then cut holes in some batting and push the figures through the batting so you don't see the board. I have a bunch of pre-built tree scenes done this way. Another way to hide the foam board is artificial snow.
Hills and background areas get the fluffy snow batting. Once lights and power cords are in place, the fluffy stuff is used to hide the infrastructure.
My plywood base looks like Swiss cheese as I'm always cutting new holes for lamp cords and accessory power. I don't try to run wires under the base batting layer but cover wires with another layer of batting where needed. Cutting holes after the batting is in place is doable but the batting has to have a window cut in it to drill the hole.
Track screws like to wind up the base batting but firm pressure on the track to keep the batting from moving while screwing in reduces the problem.
Finally, a dusting of artificial snow (the Dept 56 is best) goes on selected areas that get a lot of visual attention. Keep it away from track.
WRT artificial snow, one of our show's layouts uses a lot of it and the owners have a mini shop vac dedicated to collecting the snow for reuse.
