Many hotel rooms have windows that don't open, or that can't be opened out of safety concerns. Such rooms may be perfectly clean, but they still tend to smell stale or musty. There also can be a hint of smoke in the air, even in a non-smoking area.

I used to ask to be moved to another room, but they were all the same! I've learned to ask instead for an electronic air filter I can plug in. Overseas, I might ask for a floor fan. Both work fast and do wonders to circulate and clear the air.

Bonus: both these items provide restful background "white noise."