Hold up an image in front of the group, and ask "What do you see?"
Allow the participants to exhaust themselves trying to describe it. Then, start narrowing in. Ask them to describe specific parts of the image.Ask them to detail everything that is red. Everything that seems to be moving. Everything touching the ground. Everything smaller than a human. Ask them to clarify and detail descriptions. The more questions, and the more detailed the questions, the more information they'll be able to extract from the image.
Also, an improv game called Pointing At Things. To begin, the player points at something, anything, and says the object's name. The player then, without missing a beat, must point to another object, naming it as well. The next stage is to point at an object and say the name of thing thing you pointed at previously (Not particularly relevant to the exercise at hand, but still interesting). Finally, the player points at objects and says something completely unrelated. Most people find this extremely difficult, because there aren't any specific prompts from the environment, even ANYTHING could be a prompt.
Continuing with Improv, you could ask for two volunteers, and tell them to improv a scene, right now, without any discussion. Just go. When that fails, tell them to improv a scene about love. Let that go on a bit, and then ask them to make a scene about the love between a child and a parent. The love between a child and a parent becoming strained. The love of between a child and a parent becoming strained because the child is going to university and the parent can't let go. Keep on clarifying and refining the scene until the characters and situation are well defined. The same exercise could be done by asking an individual to tell a story, and then asking them to restart the story, bringing in the elements provided.
Cavemen is a trochee. Astronauts is a dactyl, which is why it's less funny, but still close enough to a trochee.