A.C.
I went and took pictures with my daughter (then 14 months). We did some of her hugging my tummy, her pointing to my belly button, and with both of us showing our tummys. I then put a collage of them in a frame with the caption "surprise, kaia's going to be a big sister!" I gave the frame to my husband for father's day.
For the grandparents, we gave each set one picture of her pointing to my tummy. The picture had a speech bubble (like in the comics) saying "I'm gonna be a big sister!"
For the extended family, we got lucky because there was a big family party, so I got my daughter a tshirt that said "Big sis in training" and she wore it to that event so we could tell them all at once. You'd be surprised how many people didn't get it :)
She wore that shirt if we ever wanted to tell people in person. For lost distance fam/friends, we emailed a copy of photo we gave to the grandparents.
We got the pics done at Target, and it was pretty cheap, but you could certainly do them yourself too.