When I was a kid, my friend had this pen with invisible ink. You could write a note with it and then the person you sent it to had a different pen that made the text show up. We though it was awesome because you could send notes and if you got caught it was just blank paper. If you want to relive that sort of mystery from your youth, Make has some instructions to recreate that invisible ink for the modern age.
To build your own ink jet you need a printer, concentrated lemon juice, a hacksaw, a rubber hammer, and some time on your hands among other things. The printer uses the lemon juice as the ink and to get the ink to show up you need a spray bottle with iodine tincture.
That iodine turns the paper purple but the lemon juice breaks up the iodine on the letters printed with it so they stand out clearly on the page. The giveaway for secret agents using this technique will be the lemony fresh scent and purple stained fingers.