But it turns out that the only cheapish 60 million year old fossils are small coprolites, AKA fossilised dino s-it, which wasn't quite the message I wanted to send.
So what I ended up getting was a Mosasaur Tooth, about 85 million years old, which came with a certificate of authenticity. I put that on a card that said "You may feel old - but this is more than a million times older" and laminated that, found a nice web site with an article on mososaurs that came out at a printed page and laminated that too, and stuck the open-topped cardboard box with the tooth inside a clear bag and put that in with them. If I'd thought it through properly I would have tried to get a nice clear box and stuck that on the card, but I forgot to sort it in time.
Ended up as quite a nice little gift, and not terribly expensive - about 12 quid for a moderately sized tooth, about an inch long, inc. P&P etc., plus negligible costs for printing, laminating, etc.
My sister will also be 60 next year, I think I'll do something of the sort for her too - but she's a very keen gardener so it'll probably be a plant fossil, not a tooth.
I pass the idea on for anyone else who can use it.