I really like using liquid trays for my overhead projector. I use liquid watercolor for color and then grapeseed oil and water. I also bought some tiny styrofoam balls that look pretty neat and are kind of rascally. Some other things: I have a wet tray and a dry tray. The dry tray currently has styrofoam balls, plastic microbeads, colored beads and iron filings. Very fun to apprecate how multitudinous they look.
Overhead Projection Fun