Shred some red cabbage and boil it in a small amount of water for 30 mins. Strain the amazing purple juice and then paint it onto your paper!
add some lemon juice
bleach (bathroom cleaner)
milk (actually the dregs of the whey in the natural yoghurt)
bi carb (you can dilute it first or sprinkle on as powder like we did)
drops of vinegar
other suggestions to try were water, toothpaste, citric acid, cola, washing up liquid, laundry liquid.
Have a look on my Pintrest Science experiment board for my inspiration and other ideas.