FTM here, so I don't have experience with this but I love the idea. I think it is great to make sure that your older child feels included. I'm sure it can get a little stressful for them especially when all of the attention is on their new sister.
This isn't a gift, per se, but one of my friends recently bought some scrubs off of Etsy that said Big Sister [child's name] that her daughter wore when her sister was being born. She gave it to her a couple of weeks early so that she could get excited about it. She LOVED it.
As for gifts, I would make it super personalized to what your DD like currently. Does she like to read? Maybe get a book for her from her baby sister. I recently bought a book from lostmy.name which is awesome because you can personalize it for your child's name and even pick out which animals appear in it.