The toilet tank has been an object ignored by most people as far as creative makeovers go. There's not much you can do with it but Vital Industries has some creative stick-on graphics that you can apply for some interest.
InventorSpot notes, "If you're looking for a surefire way to make a splash (figuratively, of course) in the "WC," then these nifty vinyl stickers are for you. Forget that stupid furry toilet seat cover - those are just gross. These heavy-duty, vinyl decals are sure to "trick out your toilet" with their funky designs and unusual placement. The folks at the Vital shop on etsy.com claim that these suckers will last for years, but come off easily with adhesive remover and a razor blade."