Wow, everyone has some really great ideas!
Clare, I literally started crying after reading your photo idea! I think I might have to steal this one for when I tell my grandmother when she comes to visit for the holidays in December. It will be the first time she meets DH and the idea of having my mom take a picture/video of the 3 of us would be so awesome :)
Okay, I have put a lot of thought into this topic...
My mom has known that we have been TTC, and she is the most intuitive/curious woman in the world. (My brother calls her Nancy Drew.) I wanted to surprise her, which is nearly impossible. So, I told her I was testing on Wednesday of last week, but on Monday (I had already got a BFP the week before) I asked her to pick up some photos for me at Costco. I told her they were from DS's soccer game the week before. I said, "Before you pay, make sure the colors are dark enough, they looked a little washed-out on the computer." She was very concerned about getting it right for me--didn't suspect I was up to something. I guess the lady at Costco said, "There's only one picture..?" They looked at it together and it was a picture of me holding the BFP pregnancy test with a big smile. She was so surprised! She called me from the parking lot and had me on speaker phone when she handed the photo to my dad. They were thrilled.
DH is telling his parents on his dad's 70th birthday 11/11. We are sending them a package with a card signed by all of us and "Baby Z" along with a "Book of Grandparent Names" (since they have told us they would want to be called something different than Grandma and Grandpa--first grandchild for them, my DS is their step-grandson.
As far as DS, he is 5 so once he knows, the whole world will know. I am going to wait until after my 12 week appointment. Then, on Tuesday night I will ask DS, "Does my tummy look bigger? Do you think there might be a baby growing in there? Should I take a test and see if it's true?!?" (He has been wanting a baby to grow in my belly for a while.) I will then take a test and tell him, "Okay, if you see 1 line there is no baby, but if you see 2 lines, then there is a baby..." He will see the 2 lines and tell ME that I am pregnant. Then, the next day at school, it is his "Share Day" of the week. I thought he could "share" with his kindergarten class that he is going to be a big brother rather then bring a toy or art project as usual.
And lastly, for Facebook. After DS knows, I think I will just take a belly shot and post it on Facebook around Christmas with the caption "Baby Z". I hope I will be showing a little by then--baby #2 at 14 weeks?