Making taller baseboard: Spacers were used to separate two pieces of baseboard/trim during installation. The drywall was painted before the upper trim was installed. These guys used an accent colour, but you can also use trim colour.
Better-looking return air grille. Check out another DIY return air grille solution here. Do you have a baseboard return air grille that looks like this? Not especially attractive, right? I'm pretty sure o.