Posters for the seasons and holidays can be made if someone will draw a big, traditional shape, such as a 6 foot (1.8 m) Christmas tree on a huge piece of white paper. Give people access to pieces of green tissue paper. They can crinkle the paper and stick it on with rubber cement during the first session. During the next session they can stick on the ornaments, which are different colours of tissue paper.
It will take some planning ahead for them to have access to the top and bottom of the poster. Try stapling the folded top to a dowel that can be raised or lowered.
Autumn trees, pumpkins, turkeys, hearts, Easter bunnies, flowers, flags, etc. are stereotyped themes that they still recognise and enjoy being a part of.