We bought her a dolly and a pram. She likes copying me when I change baby etc x
My daughter was 21 months too when her brother was born. She looooves babies, so I had a new baby and baby bottle/blanket for her so she could hold her baby like mommy has hers. We also did books too because she loves to read.
I did a backpack gift both times I was due to give birth. I put in a few new toys, some books, some activities, snacks, I included a little photo album to fill with pictures of them together and a little note from baby to sibling/s. It doesn’t have to be anything crazy just things you know she already likes or will occasionally keep her occupied