Aw yis. Now you're in my wheelhouse. I've managed planetariums in science centers for the last 8 years. Excited to see what you do with this.
Possible ideas for this theme:
So many fossil possibilities (dinos, whales, mastodons, synapsids, mammals, etc)
Minerals, gemstones, crystals,
meteorites, telescopes, constellation paintings,
Large Lobby displays like huge globes, dino replicas with skin, spacecraft
Anthropology related materials like old baskets/spears/hides/old dwellings
Nature dioramas with wildlife/foliage/water