You could make her cup cakes with blue or pink icing inside, and take pics/video of her eating it..
The balloon idea :) get a box and stuff blue or pink balloons in it. Very cute..we did it
My husband and I had a paint fight. My mom put the correct color paint in ketchup bottles and then we squirted each other and had pictures taken while doing it. We were going to have a silly string fight but we kept the white shirts and framed them for her nursery.
When i found out i was having a boy my mom copied the ultraound onto the computer and we photoshoped a bowtie onto his neck. And sent the picture to our family. You could do that or a pink bow on her head for a girl print it out and have her hold it. Have her hold pink or blue balloons, paint a picture with just pink or blue paint, have her hold a girl or boy onesie up, or hold a sign up that says im having a baby brother or sister. Anything would really be cute.
I like the idea if I waddling a gender reveal party though I saw that on interest as well.
I can't really get me in it because no one to take the picture. And inplus its cold outside and we have carpet in our rental and had bad experiences of silly string staining things.
Have her spray a can of silly string at your belly with the right color!
I wanted to something like the paint I saw it on interest but its exactly what my sister in law did with her two kids.
Your daughter could wear a shirt saying "I'm going to have a baby brother/sister". Something simple so it doesn't take away from the special moment.
I saw a black and white picture of mom painting siblings hands, then sibling putting hands on bump and finally the blue/pink handprints. Handprints were the only color. I think its cute