My boyfriend is serving in Afghanistan and I try hard to make sure to send him entertaining, yet loving, things to remind him that I'm still waiting for him.  For one of his recent gifts I bought a Rubik's Cube and wrote a riddle (one word on each square) and then scrambled the cube. 

The riddle was a cute "love" riddle and the answer is romantic. Plus, to prove that he has the correct answer he has to "show" me what it is.  Last time I checked he was still trying to figure out the Rubik's Cube!