High school students who are throwing a dance in the gym might enliven it with balloons and streamers. In other words, they might try to cheer the room up.
When you enliven something, you make it more lively, colorful, or exciting. Your friend who is the life of the party can probably be counted on to enliven the dullest gathering, and a sprinkle of curry powder can enliven a dish of plain roasted vegetables. The original meaning of enliven, from the seventeenth century, was literally "to bring to life," as when a doctor re-starts a patient's stopped heartbeat.