For my 1st, we decided to move back near the family, so we told them that we decided that we needed to be closer to doctors and such and that we were moving back in with them! (we didn't),
For my 2nd, we were in the middle of doing some renovating, so we told them we had decided to add on ... to the family! (for a second they thought we meant to the house :) ) Sadly, that one miscarried. :*(
For my 3rd pregnancy, we didn't tell anyone. My son's birthday party was just past the 3 months mark. so I found a card with a cute puppy on it that said 'Wanna know a secret?'. I placed a sono picture in it with 'you're invited to the next 'birth' day in 6 months! SHHH!! ;)
I cleared the fridge of all magnets and such and taped it to the front. Then I kept sending family members to get the fridge to get me things. Each would let out a little squeak and then come find me. It took about an hour for the it to get around the party ... it was fun.
For you, I would say do a picture, either Thanksgiving or Christmas card, that is 4 pics. One kid for each corner. In the 4th corner, do the pic of a present, with the tag 'Don't open until May 2014!'.