I would say go for the castle and have one of the Disney photographers take the picture. The castle is the backdrop that the average person is going to recognize even if they’re not a huge Disney fan. I haven’t been there since Covid so maybe someone who has could confirm if these photo spots are still there? (I think you have to purchase PhotoPass for these photos which is a bummer because before you could buy individual photos, but they’ll be high quality and you won’t have to try to stop a flood of people so that you can get a clear spot for your picture.)
Another one might be meeting Mickey at Town Square Theater… he’s behind a rope for social distancing right now but that could change soon.
Regarding props… this isn’t a prop exactly but I bet if you search Google or Etsy for a Disney maternity shirt they’ll have something cute!