If a girl you could follow a similar line to the roses and gift a Swarovski crystal figurine or something of that nature - some of them are limited and later can become collector’s items as far as I understand. They have very small ones which might be within the price range.

If a boy, maybe cards from another sport?

Don’t listen to all of the naysayers, aside from savings bonds being a very practical and good idea to consider, if my parents gave me a collectible item on a particular date every year I would absolutely treasure it. Perhaps not while I was moving around at university, but absolutely when I got settled in a home when a bit older. My partner has a book of Pokémon cards, and yes it’s in a folder gathering dust, but he has fond memories of searching flea markets on weekends with his dad for more. Every time he gets it out his face lights up - and it’s worth some money now.

Maybe baby #3’s gift could be something that you could do together to give the gift that additional meaning?