There are some really adorable ideas out there. I considered making one of those little robin's eggs that you crack open, confetti/glitter pops out, and you read a teeny tiny scroll that announces your news.
You could do a similar thing with valentines heart shaped balloons, stuff them with glitter and put in a note, have your hubby pop the balloon and read the note. Warning: in this situation, glitter and confetti LITERALLY go everywhere, so paper confetti might be the way to go. Glitter is EVIL in large quantities!
You could put cute little red booties in a box and a note that says, "Please save these for me! - Baby"
My DH is so oblivious it has to be like a neon sign for him to get things, but if you wanted it to be more sneaky, you could give him a bouquet of Baby's Breath and just a note that says the due date, like 11.20.2016.