Sounds like you already have a bunch of good ideas.  Trips to other local productions are always fun.   Workshops in Scene painting, makeup techniques, flat building, dance, Puppetry, Readers theater, the list is endless.   Maybe a Halloween costume creation workshop with a trip to the local resale shops. Have the kids plan out costumes and then try to figure out creative ways to make them.  Might be a good chance to do a makeup workshop too

Our local group just did a musical theater workshop, finishing up with Maurice Sendaks Really Rosie. 

My daughter and a couple other high school students acted as assistant directors.  They got experience in both teaching and choreography.  The kids had a final performance after three weeks and charged $4.00 for the show.  The kids were charged a nominal fee for the workshop as well.  This helps to bring money into the program to fund scholarships for kids that cannot afford full payments.  

They are also having a Shakespeare workshop and bringing in a local professional actor to teach fighting techniques to the teens and how to recite Shakespeare.  In the end they are going to a local production.