I've seen some cute ideas on youtube. Or maybe pinterest.com would have something.
A fun way to do it is at the next family gathering, have everyone get together for a picture and have your baby's father go to take the picture, and he will say okay in 3,2,1, smile! And then you shout we're pregnant! And then he takes the picture of everyone's reaction of shock while they are staring at you and you're just smiling, lol too fun.
If you have a family pet, you can have everyone over for dinner and then midway through put a shirt on your pet that says "soon to be big brother/sister" and just wait for someone to notice
You could send out a greeting card, I would look on Pinterest for ideas for that one.