I would like to share what I tried in this similar line.
Team Building Session Experience
I conducted a team-building session for a 30-member team. This session had a clear set of defined objectives. We had activities around each development area (which I identified prior to the program), and after each activity, I asked the team to come up with action plans for that particular development area. Then, I formed a focus group (4 members), giving responsibility to these people to ensure that these action points (top 4 in the list) are implemented and practiced across the team. Twice every week, I had a one-hour session with the focus group to check on the progress, observations, and support required for the team.
I did try to conduct a mystery audit to find out and document the effectiveness of the program, which I couldn't complete 100%. However, this did yield benefits as I received good responses from the management on the change in team performance, especially in the identified development areas.