Try to find an attractive air return grille at the big box store and you'll be in for a surprise. They won't have any. At least, nothing affordable. Hillary, from The Friendly Home, found a way to DIY an attractive vent cover using some decorative sheet metal she found at Lowes. She trimmed it to size and then framed it out in pine 1 x 2's. Genius! Although she caulked her vent in place, check out the comments in her post for ideas on how to make it removable. Click here to read more details about the project.
A better looking return air grille [The Friendly Home]