Mantle decoration/draping

I could imagine something like this being really pretty. You could do a floral garland like what's pictured and also add some pretty, chiffon fabric following the same path.

ETA: you could also put some candles of varying heights inside the fireplace to make it extra pretty.

If it was me I would light a fire in it. Do you have a picture of it? ETA it's not working. Sorry. Wine.

It's called pipe-and-drape. Call local rental companies and ask about it.

I think it would be so romantic to fill it with candles of all different heights, probably less expensive than draping it with flowers and fabric.

Here is an idea

You can do something like this.

I think if you are planning this then it is important that you should follow all the fireplace safety tips to keep you and your family safe. Be sure to keep the area around the hearth clear of debris, decorations and any other combustible materials and have your fireplace inspected and cleaned by a licensed chimney specialist Nassau county beforehand.