Yep, I’d go for growing a nitrogen fixing plant like peas or beans - plants that give something back. If you cut down egg boxes you fat little plant pots which you can grow seedlings in and can then plant straight into the soil as they biodegrade.
Maybe also environmental “pledges” decorated with drawings or collages.
Junk modelled “green monsters”, using your recyling one more time before it is recycled (as a children’s worker I used to do this with all ages from toddler to teen and they *all* loved it!)
Making anything (instruments is good) with “found” objects - spend 15 mins outside the building looking for stuff for shakers etc, provide string or twine and then spend 5 mins at the end singing with homemade shakers and stringed “instruments”.