We did a variation of wax resist drawings with a candle (we used a tea-light) rather than coloured wax crayons. It added a element of excitement to the activity as Finn couldn't see what he was drawing, but could feel his way around the page and envisage his drawing. I then gave him some watercolour paints and he magically revealed his drawing. A strong even colour using an ink based wash (made from food colouring or edicol dye) would have been more effective in hindsight.
Finn and I also created secret messages and drawings for each other. After swapping, we painted over the paper to see what we received.