A fun activity to do with siblings or at a baby shower/blessingway is decorating onesies. You can find a five pack at the store for around $10 and grab some fabric paints (about $1/bottle) and get creative!
Here's a beautiful collage our birth photographer assembled of what our older five made for baby #6:
The older ones did it on their own, the middle one asked me to help write out and let him trace something, then there's some modern art from #4 and the littlest we traced his hand and wrote the instructions "Tickle here" on the baby's belly. 🙂 It was a great way to get the kids involved and excited about the pregnancy.
Another sibling art project that was highly amusing, I stood in a dark hallway and Kit shown a flashlight at me while the kids traced my silhouette on paper taped to the wall. Then they each drew a picture of what the baby was doing inside of me - hysterical!
And of course, the belly cast is a delightful mess!
Other fun idea, let the kids fingerpaint your belly, though I advise you to have a lot of wipes on hand and be ready with the camera!