There are several different types of pine as the following descriptions show: Pine - Eastern White (Domestic): This light colored wood resembles straw in appearance. Soft straight grained and even textured wood weighing 2.2 pounds per board foot. Works very easily with hand and machine tools. Glues well and takes stain, paint very well. The most valuable softwood in North America, as it can be used in almost any piece of furniture or most any form of general carpentry. Pine - Ponderosa (Domestic): Light orange brown to reddish brown. Fine to medium texture and straight grained. The most resinous of the pines. Easy to work with, although finishing can be tricky due to its gumminess. Uses include furniture, construction, window frames and interior trim. Weighs 2.2 pounds per board foot. Pine - Sugar (Domestic): Cream to light tan in color, it has a soft fine texture and straight grain. Large trees result in better width and length averages than other pines. Its characteristics make it especially suited to the pattern industry. Weighs 2.6 pounds per board foot. Pine - Yellow (Southern US): Creamy white sapwood, yellow red to reddish brown heartwood. Very resinous, conspicuous growth ring figure, course texture. Weight 4.21 lbs./ft. Dries well with little degrade and is stable in service. High bending and crushing strengths, high stiffness, medium resist to shock. Holds screws and nails firmly, glues with out difficulty, takes paint and finishes satisfactorily. Plus, some hardwoods like aspen, firs, or birch are sometimes mistaken for pine.

Have them fabricated to 10" and have the installer cut them to size during installation. This would be the normal procedure.

We have this situation in our recent remodel bc there was a weird space in the wall (AC return, other stuff we didn't want to re-route) that forced us to have some wasted space behind the vanity. We could have just closed it up with drywall, but instead we decided to put some shelves in there. We still have some empty shelves, but this is what we have done so far: candles and bath products. For the remaining shelves, probably we will put a framed photograph, a nice piece of coral or large shell, or some pretty pottery. Below we don't have an open space like you do; it is a cabinet door. If you don't already have a good spot for a hamper, that might be a good place for one. If you already have a place for a hamper, you could put a smaller "dry-clean" hamper there for things you want to separate to take to the dry cleaners.

Shelves above the toilet would be good. You could store things that guests might want easy access too-- wet wipes, tissues, extra toilet paper, air freshener, etc. You could get a "space saver" but you would probably want to raise it up higher than the default ones usually sit. I find the store-bought ones tend to sit too close to the top of the tank & interfere with maintenance. You want something that won't block the removal of the tank lid & won't get in the way if you have to lean in to change the flapper or take apart the flush valve assembly. You want to be able to lean forward enough without hitting your head on a shelf. In our main bathroom we have custom built shelves about 5' from the floor. They go from wall-to-wall. Although, it's not an alcove so there is a vertical piece where the toilet paper holder & a small shelf were added. The small shelf holds reading glasses, nail clippers, tweezers, pens, pencils etc. We also have a magazine rack on the floor that holds various reading material & fill-it-in books (sort of like crosswords). Across from the toilet we have a storage cabinet that holds extra toilet paper. The window looks fine without any curtains or shades. Here are pics of the setup at my house (please excuse the mess-- I've cleaned it up since then but haven't gotten new pics yet). To spice things up a bit more, maybe you could add an accent color to the upper wall above the toilet or add some chair rail molding and/or beadboard at the bottom. Here's what some floating shelves could look like if you added them (these ones have LED puck lights):