I have participated in both in-home fostering and short term fostering.
Although both my in-home foster dogs would be deemed "foster failures" we don't see it that way--we are blessed every day!!
My short term foster experience consists of once a week taking dogs for a truck ride [they get to be "front seat dogs"] to the local park that has amazing trails through a forest--they get to sniff for wild-life mark every thing they want and BE A DOG. After this we go to the local McDonald's for plain cheeseburgers and then back to the shelter. The whole outing is about an hour but they often fall asleep on the way seemingly relaxed and contented.
Although my perception of the benefits for the animals is 100% subjective I know there have been studies that demonstrated positive physiological responses [sadly they seem to go away when returning to the shelter..].
Interestingly what I have found is how this experience has benefited ME. In my other life as a physician there is a lot of stress and these outings melt this away!! Fostering is mutually beneficial and we should remind ourselves all the time of the benefits we get as "foster parents".
I also am happy to say the others have now started doing the "doggies day out" so sharing my experience has multiplied the benefit to animals in our shelter.