Place a shallow dish of baking soda behind the toilet to absorb bathroom odours. You will achieve the same results using a shallow bowl of vinegar. Note that these tips do not mask the smells they actually absorb them.
More ideas for the restroom are...
Thread potpourri and hang it on the wall. You can use this to border pictures or the mirror. It's decorative and makes the restroom smell nice.
One of the most effective ways to mask unpleasant bathroom odor is also one of the easiest. Simply light a match or two, and you’ll camouflage any smells. Keep a few matchbooks in your bathroom for emergencies.