Are you bringing a digital camera with you?
If you are, make a big sign that says, "We're pregnant!" on it and smuggle it into the park with you. (Even if you have to fold it.) When you have a moment when the inlaws aren't around, have someone take a picture of you and your DH holding the sign. Then throw the sign away. At the end of the day, take a moment to "review" the pics from the day. See how long it takes them to catch it!
We did something like this with our first pregnancy, only we were at this beautiful scenic overlook at Mt. Rainier in Washington state. We got such a kick out of showing everyone the "pics from our trip." What a hoot! And it's so fun to look at that pic . . .
Congratulations!!! Enjoy spreading the news, however you choose to do it!