I wasn't following this recent thread so don't know if this idea was
mentioned. In the latest issue of Expression magazine one of the
contributors (Sherrill Kahn) shows glazed ceramic tiles she covered with
one of several materials: molding paste (which is sold in the acrylic
paints section), heavy gel medium, or caulk. The caulk sounds like the
cheapest, and I wonder if a tub of spackle would work as well. She
sanded the tiles with 50-60 grit sandpaper so the material would stick
to it. While the paste was still damp you press a stamp or other object
into it to create textures, then let it dry. Afterwards she painted
them with some of the expensive crafts paints. The layers looked really
good in the magazine. The whole project sounds do-able and relatively
inexpensive for several classes' worth of students.
There is also a project for making a miniature star book to hang on a
tree; this also looks fairly do-able with older kids.