Regarding smoke alarms, I got quite a clever little one that screws into a light fitting and then the bulb goes into it, and it is hidden by the lampshade (although the lampshade is open enough that smoke can get in were there any smoke).
It has a rechargeable battery and is charged when the light is on (light has to be on minimum one hour a week) so battery will hardly ever need changing. You test it by flicking the light switch once and shut it up (when it goes off when you burn the dinner) by flicking the light switch twice so no getting a chair to stand on etc. And it saved me drilling my ceiling.
It does work. Not that I, er, burn a lot of food or anything...
£19.99 from Maplin.