Dave, Would you have any problem with having students design or draw
their own coloring books? They could do some ink, pencil or pen line
drawings of different things. Photocopies could be made and distributed
to all the other students in class, so that each of the 25 students would
have a copy of every one of the other 24 student's drawing. Then let them
color them their way, of course. They could add line drawing to the
others' works if they choose.
All 24 students works of the one student's designed drawing selected
could be exhibited and class critiqued.
They could be unsigned and then have the students try to guess who drew
this one, but who colored that one, etc..