When using the bathroom in someone’s home, be certain to leave the bathroom as clean as you found it. Put the toilet seat and cover down when you are done, and pull the door all but shut when you leave. If you’ve left the bathroom a bit smelly, you should make sure to use the fan or crack a window (if reasonably possible) to air things out a bit.
If there is air freshener present and you wish to use it, use only a small amount. A candle or match (if available) may also help to dissipate odors.
And remember, when it comes to the perennial debate between men and women about the toilet seat, the answer (for everyone) is to leave both the toilet seat and cover down. It’s just polite.