A Beautiful Mess recently had a great post about making homemade stove simmers. I tried my own version the night before Thanksgiving and it made the apartment smell delicious. I used fresh cranberries, orange peels, lemon slices, fresh rosemary and a cinnamon stick. I kept it simmering throughout the whole night, adding more water when needed. At one point I threw in some fresh ginger and apple slices. I love this idea because you can really make it your own- you can simmer just about anything that you think might smell good.
This is a similar idea for the fireplace. I'm going to make a few to take to my in-laws house since they have a real fireplace and we only have a gas one. It's going to smell so Christmas-y!