To romp is to play or cavort. On a busy day at a playground, you can watch happy kids romp.
When you romp, you don't entertain yourself quietly, but instead run around laughing or otherwise frolic with energy and enthusiasm. You can also call a wonderful, fun activity a romp: "Her party was a total romp!" The verb romp probably comes from the now-obsolete ramp, "to climb" (for a human), or "to stand on hind legs" (for an animal).