Many children's mystery and adventure stories have a secret message in the plot - and here's some paper that the kids can use for their very own secret messages!
You could try writing your secret message in code. Or how about trying the lemon juice trick? Use a clean nib or feather point - or perhaps a cotton bud - dipped into lemon juice to write your message. When it has dried it will be invisible. To make the message appear, warm the paper carefully over a candle flame - STRICTLY ADULTS ONLY!