Feel free to steal (borrow) some stuff from my website at
http://web1.lakefield.net/~dday/ref.htm. I'm the VP and football chair of our local association. We recently adopted new rules about our in-season sports that requires at least 60 minutes of educational material at each meeting.
Few notes of caution - these presentations are from past seasons so be aware of potential rule changes. I suggest you not use the presentation about keys for 5-man crews - it was heavily edited for time. There is a much better version on the national federation web site.
Another fun activity - have everyone bring their whistle and take turns blowing the whistle, blowing the whistle and stopping the clock, etc. You'll be amazed at how bad some people look! Sorry, I mean you'll be amazed at how much people can learn and improve their mechanics. Practice ball relay - one thing that can make a crew look really bad. Again, many refs didn't play football and don't know how to toss the ball underhanded.
Reminds me, I have a clinic to prepare for and football meetings to plan! The season is upon us.