Write your note in baking soda solution.
Allow to dry.
The note will be barely visible (though not invisible).
Paint grape juice over the note to make it visible.
(The grape juice turns a different colour when covering the basic baking soda solution).
Other ideas for invisible ink:
lemon juice invisible until heated
Ex-Lax tablets in water (contain phenopthalein) to write. Add baking soda solution to visualize.
The School at Columbia