When my FIL turned 90, his children got together and gave him 90 gold dollars. No, he didn't need it, but he was tickled pink - er, gold.
When my mother turned 90, her friends presented her with a list entitled, "What's Good About Being 90." She loved it so much that I have used it since for other people who hit that age - modifying it to their personalities and experiences. Some of the possible items are:
You can sit back and watch your family tree grow taller and wider.
You have more time to hear about your exceptional grandchildren.
You are wise enough to remember that "How are you?" is a greeting, not a serious question. So everyone is glad to greet you!
89-year-olds envy you.
You can have your 100th birthday party at ANY date you choose in the next ten years. Next week's fine.
People go out of their way to help you with anything you want - you can even stop traffic and cross the street wherever you like.
Your to-do list is as long or as short as you want it to be.
If you forget things, nobody minds. They're just amazed that you're still around!
You can eat chocolate cake and jelly beans for breakfast with nobody lifting an eyebrow.
You can flirt with younger men (those dudes in their 80s).
You have the excitement, and sometimes the pride, of having lived through history that the rest of us have only read about.
In roman numerals, you are XC-llent!
You can also add a few good quotes and poems. Make it serious or funny or sweet (or all of the above), as you prefer. Make sure it's read out loud at the party.