Treasure bag gameWorks really well for multi-week groups...
I used to carry a small fanny pack with me, with two comparments.
Compartment one housed a series of drill/activity cards. These can be tailored to specific age groups.
Compartment two, the bigger one, housed the treasure - cool pencils, stickers, gum, mini-paks of play-doh, fruit roll-ups, kinder surprise, balloons, parent-approved candies, etc. Small, inexpensive items from the shelves of your local dollar store.
Each person in the group would have a chance to reach into the bag and pull out an activity or drill card. After I explained/demo'd the drill or activity, the person who chose it has the chance to lead the group through it.
Once they have successfully (which is always, simply trying equals success) led the group through an activity, they get to reach into the bag again - this time on the treasure side, and feel around and pull out their surprise reward. Make sure everyone in the group gets the chance to lead!!
It'll cost you a few bucks, but it's pretty darn fun, keeps everyone involved and motivated.
Don't know if it'll help you in your situation, but it worked for me when I worked with a group of "Nancy Greeners" for a couple of seasons a few years back. As time went on, the tasks got more and more challenging, and I had to implement "bonus" tasks that were so tricky, everyone who pulled these specially marked cards got a bonus treasure grab.