What a great idea:

To go outside Diesel and Stella have been taught to ring the cowbell hanging from the door handle. One belt with a paw or a nose, the bell rings and the well trained door attendant (me) obediently opens to door for the two to go roaring off around  the yard. Mind you they always ring the bell when I am talking on Skype or busy preparing food, anytime I am not paying them attention, but they are not too demanding.
Diesel's trips outside are usually pretty short particularly if it's cold and snowy, but Stella likes to  sneak out toys, bones etc and re-discover them when the snows melts a little. She will spend quite a time out there snuffling around in the snow, chasing squirrels or barking dementedly at Desi next door and rushing up and down along the fence.
To come back inside it's a sharp yip, which gets sharper the longer I ignore it!