We debated whether or not to put in a 90th anniversary entry partly because we don’t think any couple has or will in our lifetime make it to such a milestone but then we thought, damn, if a couple made it this far and there wasn’t something to honor them, we couldn’t live with ourselves.
Traditional 90th-anniversary gifts include stone.
Most websites state there is no specific flower for this anniversary, but some suggest purple flowers. These could be incorporated into your theme. Frankly, if your giftee has made it to their 90th anniversary, they likely don’t need or want any material gifts. Instead, consider gifts that brighten their day or can be dedicated to them.
Contribute to a stone bench at a local park on their behalf.