Here are three experiments to get you started.
Mix equal volumes of baking soda and water together. Using a cotton bud write a message on some paper, and leave it to dry. When it's completely dry you can paint the paper using a brush and grape juice. The message will appear a different colour from the rest of the paper. This is because the baking soda reacts with the acid in the grape juice. The message will appear more clearly if you use concentrated grape juice.
Make some red cabbage water by boiling red cabbage in water for 30 minutes.
When its cool, drain the water into a jar. You can use this water to reveal messages written in acids or bases in much the same way as you can use the cabbage water to work out whether something is an acid or a base.
Use both lemon juice and bicarbonate of soda water to write a message on a piece of paper. Use the cabbage water to reveal a two-coloured message.
You can make this into an art project where the invisible ink is used to paint a picture which is revealed by painting over the page with the cabbage water.
Finally, use a cotton bud to write a message with lemon juice. Leave the message to dry. It's possible to make the message appear by heating the paper using the heat from a light bulb - but, this can be dangerous, as you may cause it to burn. Make sure you follow relevant safety procedures if you choose to do this as a demonstration in the classroom.
A safer alternative is to sprinkle the message with salt before the lemon juice has dried. Leave it to dry and then brush off the salt. The message should be invisible. Now rub over the paper with a pencil or a wax crayon.
The message will appear.