Just a little fun...
I just finished the first retreat that I had to plan. For us, the number one thing to remember was free time was FREE TIME, try not to dictate what they will be doing.
Secondly, I had everyone bring a joke - I really had no particular reason, but in the end it helped. It helped break the ice, it helped form new bonds, and they were just fun.
So here's my joke
What kind of bees give milk?
BOObies!