We sat my parents down, telling them I didn't have a recent picture of them with our son then as they were smiling and posing, I said, "I, 2, 3... I'm pregnant!" waited a beat and took the picture.
It was AWESOME. They were shocked at first and then didn't quite believe us and then started crying. (It took 3 years and IVF for our first child and no one new we were trying again and back in fertility treatments for this one.)
Also, I had set up the video camera where they wouldn't notice it, so I got it all on video too!
After we had told all of our immediate families, I posted the picture of my parents on Facebook and said, "Let's play a game: Who can guess what we just told my parents in this picture?" Everyone pretty much guessed right away but again were in shock because no one thought we'd have more biological children. (We're trying to adopt as well.)
Here's a link to the photo.
http://nealandcarriesadoption.blogspot.com/2010/10/tellin...
I am so glad that we announced it this way. My dad said it as the best surprise he's ever had and he loves watching the video over and over again reliving the moment :)
For my husband's parents - who live in Nebraska - we sent them a webcam, saying we wanted to start having weekly webchats with them so they could talk to their grandson. We ordered a tshirt from cafepress.com that I made that said on the front "All I asked for was toy trucks for my second birthday, but..." and on the back, "My parents said I'm getting a sibling instead. Baby#2 due on ..." (My due date is my son's 2nd birthday!" They were shocked as well and we got it on video, too :)