Christy, you didn't say how big your nativity sets are, but if they're not too big, you might want to elevate one of them on a box or stacked books, which could be covered by fabric. I'd use a cloth napkin. Then add greenery and pinecones. Maybe votive candles. One year, I used brass candlesticks with white candles, a lot of mixed greenery, and all my star ornaments, randomly placed, adding some sheet music stars that I made. (I'm a musician.) It was one of the best mantels I've done. You might be able to incorporate some of those ideas with your nativity sets. I wouldn't let it get too busy, though. You don't want anything to detract from your unusual nativities. My mantel is painted white, though I've never had anything bleed on it. You might want to try cutting a piece of felt the same size as your mantel, and try some artist's or painter's tape to hold it in place. I don't think either would damage your wood, and if the fabric isn't fastened in place, it may slide off, taking your decorations with it! I'd read the directions carefully, though. Good luck!