Plug in the crockpot and you have a warm, comfortable smell wafting throughout your home that isn't overly pungent like some of those expensive wall-outlet units.
I keep a small simmering potpourri crockpot out on my kitchen counter and fill it half full with water.
Just be careful to watch the water level of your crockpot as the heat will cause evaporation.
If you don't have a potpourri crock, any crockpot will do.
Crockpot cooking is another great way to make the grocery dollar stretch a little further and give you more time to relax when you come home.