We have a very large cold air return in our living room. Seriously, it's close to 24" square! You can see it almost immediately when you enter our house. Several years ago I saw a Pin on Pinterest that linked to friendly-home.net, showing how to replace a standard ugly white grill with some trim and a piece of sheet metal from Lowes in a union jack pattern.
I showed my ultra-handy and talented husband the link and begged for him to make a better cover. While he was not enthralled with the idea, he agreed to make me a cover. He did not use the directions provided on the website I showed him since their setup did not work for our situation. We ended up with a pricy (the three foot by two foot piece of sheet metal runs close to $30) but attractive replacement cover that I absolutely love. It's so much better than the standard cover the HVAC guys installed!
This is one Inspired by Pinterest project that totally worked.