Depending on the dog, I find a fun trick is to get several different small stuffed animals and teach them the names of the animal. People are SO impressed when you tell your dog, "Get your blue elephant" and he does!
An even better trick is to ask them, "What kind of animal goes 'moo'?" and teach them to get the cow. The trick there is that the animal sound is the actual command; "Moo" means go get your cow, "Cluck cluck" means get the chicken, etc.
A whole different dimension comes when teaching colors. Some dogs don't seem to be able to do it, but a lot of them can. I start with yellow, blue, and red tennis balls. Then I scent them all with different smells, usually just things I have around the kitchen. They learn easier which ball you mean when they're scented differently. Then, when they're reliably picking the right ball, get a new set and don't scent them. It can be really tricky to teach, but it's a great way to show off how smart your dog is!