Well, Ken, as much as the riding thingy looks faboo, I think maybe that would be better suited for YOUR birthday!
How about a card for each year you've been together? In each card, put a gift card relating to that particular year's events.
Sierra's right on the flowers. But, rather than just going and buying a bouquet, go to the local farmer's market or something similar, where you can pick out her favorites and make your own bouquet.
And, while you're at it, make sure the girls handmake cards for her. NO PREFAB cards!! And, make sure the dog gets her something too.