Have your husband "buy" and "give" you a maternity outfit for Christmas... then open it in front of everyone. Or, better yet, "mismark" who it's for, have someone else open it, so everyone's really confused, and then claim it!!!
Buy your close family members childrens books for Christmas-ones that were your favorite, or their favorites, or have sentimental meanings. Then tell them it's to read to your baby!
Send poinsettias with a card that says "Happy Holidays from Jane, John, and Baby Doe" (only use your names, obviously!)...then wait for the phone calls to flood in! One of my coworkers did this!
Bring Christmas cookies to work or family/friends houses-only frost them in pink and blue. Slyly leave them on the counter, and everyone will get really excited!!! (this is a tradition in my office-we know someone's pregnant when there are pink and blue treats in the break room. Half the fun is guessing!)