Just make an issue of refusing alcoholic beverage on Thanksgiving and let them jump to conclusions. Or propose a toast to July 22! Good luck!
Oooo, this sounds fun. Do you have an ultrasound picture? If you do, you could make copies of it and put them in little thanksgiving cards for everyone and hand them out at the same time. If not, you could take a picture of a positive pregnancy test and put it in a card. Or, you could make a toast to a safe and happy and healthy nine months, like Grandpa Viv suggested - that would be really nice. It would take a second and then everyone would catch on and get really excited. What about buying a little bib or onesie that says "Grandma/Grandpa/Auntie (etc.) Loves Me" and then wrap one or two up and give them to the respective few individuals, who will then freak out? My cousin did that and it was awesome - everyone celebrated. The pie thing is also cute - you could write "Boy or Girl?" or "Guess Who's Expecting?" - fun fun. I am probably rambling now but this sounds great! How exciting for the two of you!
Okay, sorry, its me again. I thought of a few more ideas - what about baking some sugar cookies (that you don't show anyone until dessert time) and frost half of them pink and half of them blue? Or (if you are a crazy picture person like me or you have recently gone on vacation or got some new furnishings, have pets, etc.) you could bring pictures to the gathering to show everyone and then mix in your positive pregnancy test photo or ultrasound photo.
have everyone say what they are thankful for and you go last,surprise!