What cool trick should we teach our dogs?

Badger and Mushroom have been showing a lot of improvement in our Control Unleashed class, the third of which we attended this past Saturday. I've been handling Mushroom while L handles Badger, so most of my observations are on Mushroom. She was easily distracted by the scents on the ground, so for this last class, we gave her higher value treats (microwaved hot dogs), and she responded well. As a recovering reactive dog, I'm proud of her for not reacting to the other dogs in the class, including a little Schipperke who occasionally barks at her. She's also seen a few dogs in the parking lot of the training school (which is attached to a dog shelter), and she was interested but not barking, pulling, or whining. Badger has had trouble staying calm during class and will start whining, but he seemed quieter than he was in the last two classes.

Slimy, yet satisfying

At one point, the trainer had us practice listening skills with our dogs. We were to call out different cues at random so the dogs would have to pay attention to which action they did. They had to be cues that didn't involve a hand signal or an object, which narrowed our list to just "sit" and "down". (Perhaps this was helpful for Mushroom because she already has trouble distinguishing the two.)

Some of the tricks that the other dogs in the class did were "speak", "spin", and "sit close" (which makes me assume there is a "sit where you are" cue to go with it). I think it would be nice if our dogs picked up a new trick for this exercise, and just in general. (Well, maybe just Badger. I don't want Mushroom to be too confused.)

Mushroom slept like this through the rest of Saturday

The question is - what trick should they learn? We've avoided teaching cues for some behaviors that we don't want, such as pawing, standing on hind legs, and barking, because we've been afraid of reinforcing them. That leaves tricks that the dogs don't do as part of their normal behaviors, such as spinning in a circle or sitting in a particular location, and we have no idea how to teach those.

Does your dog know a cool trick? How did you train them to do it?