I haven't seen a guy in the trades that doesn't love hard hat stickers. You could try to incorporate them into your program. There are some training videos out there, but hands on is always best.
We did some training on weekends and after work, but it kinda went south as we grew and didn't have the time. I really wish we focused on it more now though.
I think at a minimum it needs to be enjoyable. Maybe training through the year and then contest based at a company picnic, i.e.: who can burn a steel beam the fastest, cut concrete the straightest, who can drive a truck the longest without the radio dying

.
It all seems like a lot of work, but definitley worth the investment.
Now you got me thinking!!