Amazing!!! Your guest will think you spent way more on this then you actually did!! :) This is what you need: Glue Gun, Scissors, River Rocks at the dollar Store! and Shelf Liner (or mat) in a matching color - @Debbie Benton-Baker I love this idea!
I really like this yellow. Or if you like the empty frames idea but not for the mantel, I could see it looking cool hanging on the wall somewhere, maybe over the couch or on the wall to the left of TV. You could do all white, or colors, or whatev you want