Half the fun, is just seeing what will (or won't) make it through the mail. My most recent postal adventure, is a foam paper fortune cookie. It's really easy to make, and only takes a couple of $1.00 stamps to send.
A simple, step-by-step photo tutorial is available at SheKnows.com.
Send someone an extra-special Valentine this year! They'll be so surprised when they open their mailbox to find one of these inside.