The exhibition at the fair was intended to give visitors a (select a word) of what life might be like in twenty years.

The exhibition at the fair was intended to give visitors a preview of what life might be like in twenty years. This is the best word to use because it has a literal meaning in the context of the sentence, leaving no doubt about the main idea. Other words that could be used figuratively are taste, peek, snapshot, glimpse, hint or suggestion.