Ask the team. What do they think is a good use of their time? No one likes to sit idle. If they were on a heavy schedule to meet the product launch, some downtime is useful to regenerate creativity, energy and focus. Have they documented all the processes used? Have you conducted a post-project review of what worked and what didn't and what could be improved next time?

These are some things for a temporary lull, however, the bigger question is how to engage your team when there isn't work to do. Can the team handle more projects?

Good luck!