The day my oldest(11yo) went riding with me on her new to her 650c road bike. It was fun to watch her get comfortable with the cockpit and settle into the drops as her primary position.
Second would be the last day of RAGBRAI. The teens I rode with that day had wanted to ride in one of the pacelines all week and were told no over and again by mentors for all thr obvious reasons.
it was a high mileage day but not too hilly and i agreed to let them stay on the end of a couple pacelines during quieter parts of the day's ride.
it was a blast to watch the kids keep pace and focus in on what was happening all around them to anticipate safety issues.