What about a Montessori-esque environment with more advanced materials? Basically setting up a lot of low shelves with activities that the kids can help themselves to like science experiments, maps, books, art supplies etc. If you just have one of each activity then the kids have to learn how to share or work together on the activity.
I think it might save you a lot of time/effort because herding a bunch of preschoolers to one activity at a time might be a near impossible task!
The other thing I was thinking of was lots of fun outings (not sure where you live?) but maybe you could round up some parent volunteers and take them to places like the zoo, art museums, natural history museums, a farm, factories etc.
Are there any parents that speak a second language in the co-op? Could it be there day at least once a week and everyone needs to speak that language? Maybe even switch all the posters/books over to that language on that day too. Songs in that language are always a must (and lots of fun too!).
