It's good for a foster dog to have a couple of tricks up her sleeve.
Tricks are fun to teach, fun for the dog to learn, and go a long way toward impressing prospective adopters with how clever and adorable your foster pup is. Many people are pleased but not especially awed if an adoptable dog knows Sit, Stay, and the rest of the Obedience 101 curriculum; there seems to be a sense that dogs somehow should know these things automatically. These same people are much more impressed if the dog can wave hello or spin on cue.
It isn't any more difficult to teach these tricks than it is to get a Down (and it's usually a lot easier than getting a reliable recall), but a dog who can do them has a big advantage with adopters. If your foster dog happens to be a "less adoptable" animal -- older, bigger, special needs, or of a breed that has negative associations in the public mind -- then he really needs every edge he can get. Since the end goal here is to help your foster dog find a good home, it's worth your time to teach a couple of cute tricks.
The best tricks to teach are the ones that your foster dog is most likely to be able to perform reliably on cue in front of an adopter. Many dogs have difficulty doing the Down-based tricks (Roll Over, Play Dead, etc.) in unfamiliar environments, because these tricks put them in vulnerable positions. On the other hand, most dogs feel comfortable with Sit-based tricks (Say Hi, Shake, Gimme Five, Sit Pretty) or the ones that involve simple movements (Spin, Bow). My suggestion would be to keep your foster dog's comfort level in mind and choose tricks from the latter category.
Also keep your foster dog's physical limitations in mind. Puppies under a year old should not be asked to do jumping tricks (it causes too much stress on their still-growing bodies) and very large dogs will have difficulty with some of the tricks like Sit Pretty. Senior dogs may have difficulty with some movements.
Petfinder Trick Tutorials:
(I don't use this method of teaching the Shake/High Five/Wave set of tricks, personally; I start by holding a treat in a closed fist and click/treating the dog for pawing at it, then shaping from there, rather than molding the dog's behavior by taking his paw first. It's a small difference, and either method works fine, but my preference is to let the dog think it through for himself entirely.)
ExpertVillage Trick Tutorials:
A quick caution on Spin: a very few dogs, mostly of herding breeds, are prone to obsessive-compulsive spinning or tail chasing. Once they get started, they can do it obsessively for hours. If you have any suspicion that your foster dog might be one of these, don't teach that trick.
If you want to expand your foster dog's repertoire beyond these standards, two excellent books to consider are Kyra Sundance's 101 Dog Tricks and Mary Ray's Dog Tricks: Fun and Games for Your Clever Canine.