Before I get a lot of people saying 'I just said I'm pregnant [hmmm]' this is at the request of DD and DH.
We knew DD would be excited as she had been asking about having another little sister or brother for a while so we told her in a creative 'surprise' type way during a day at the beach, she was ecstatic and loved that we told her that way, our youngest was nonplussed as didn't have much idea what was going on! She knows for now it is to still be a secret and wants to plan a big 'surprise' for her Grandparents to tell them. She loves this sort of thing, she often wants to surprise Daddy just for coming home from work and most of her surprises involve jumping out from behind curtains. 
DH has said that we should help her with her surprise as otherwise it is liable to end up with lots of leaping out at people, dancing and dressing up and nobody will understand what is going on, then she will get frustrated that they dont get it and we will have to tell them and she will be disappointed. (That makes her sound like a little diva - she really isnt at all!) Anyway DH has said we should help her plan it, but that he has no ideas, so I am wondering if any of the lovely mumsnetters could come up with a simple way she can do what she will consider a big surprise but keeping it very simple.
I did Google, but almost everything I saw was either in photo form ie for doing actual paper announcements, which I dont want, or was about announcing a first grandchild and we just told them normally then, Im not so much one for leaping out from behind curtains at people as it appears my daughter is.
Just for reference Im not a fan of handing people a pregnancy test to announce it I dont want to give her or grandparents something I have peed on to hold, I know they dont touch the peed on bit but it still just makes me feel a bit ewwwww.