I have made the easiest air fresheners and they are amazing. Subtle like I like, and cheap.
All I did was take a small mason style jar, fill it about half full of baking soda. I put in about 10-15 drops of my favorite smelling essential oil and shook around a little while.
I took some card stock and cut a circle to fit the opening. poke some small holes for the scent to to come through. To hold it on, screw the outer lid ring on over the card stock. And all done. I made a few to keep in the bathrooms over a month ago using lavender scent, and they are still going strong.