I think that we all know the answer to that question. Most managers would reply that they had their greatest and most memorable learning opportunities at work. Combining daily work with learning in the shape of Action Learning is probably the fastest growing area of executive development today.
Why? Because it translates theory into practice and learning into outcomes; fast.
Action Learning means that internal teams tackle real life challenges / opportunities as part of a learning program. As they learn leadership they practice it in parallel during the program thereby testing theory in real time.
If you want to develop young and upcoming managers sending them to yet another seminar may not be the answer. Managers today need to be challenged during the learning process and be permitted to learn from each other whilst creating added value at the same time.