You could just route those mouldings straight into your rails with an ogee bit and not have to worry about getting the extra pre-made bits and fitting them all in, if you want to go down that path. Though yes, you could cut a design into the sides or cut something from another piece and glue it on.. or if you're painting, just paint something in it. You could throw some metallic paint over it and clear-coat over that, you could even glue on some wallpaper, whatever.
That said, it's for a bathroom, so unless you already have some pretty ornate stuff in there (and from your desire to paint it I'm guessing that's the case?), I'd just keep it clean. Slab sides of a decent quality ply, even down to 1/4" would be fine (it's for TP and stuff, not an engine hoist), and a lot easier to just wipe down without worrying about it becoming a trap for dust and crud that'd build up in a humid/wet bathroom. If you want to pretty up the corners a little, glue a solid 1/4" thick strip of timber across either side in an L, it'll give a little extra support as well as hiding the end grain, plus you can extend them down to make legs. Shelves much the same, a ply sheet resting on some little rails, all glued together and into the sides. Grab a semi- or full-gloss white spray can, and another of clear acrylic to topcoat it. If it's going to be in the same room as a shower/bath, it's going to be in humid air and possibly collecting condensation, so it needs to be thoroughly sealed.
One strong suggestion on that note: get some plastic feet attached to keep any timber (even if it's painted) off the floor. Any sort of moisture like that would ruin it quick. If you do the 'corner trim legs' option, you might need to put something solid in down at the bottom of them to serve as an attachment point for them.