What's a good present for someone who's turning 50?

I hate getting Gift cards. Why would you want to limit the store that the person can spend the amount of money that you gave him? Maybe people feel weird about giving just money? Don't.
Think about it. How would you feel if someone gave you some cash? I can't fathom anyone who would be against it. Just put it in a birthday card.
But thats just if you are considering the giftcard idea.

Personally, if you know anything about your boss then get him a book. Books are really touching items if you get one that they'd have an interest in reading. People tend to keep books for their whole life in most cases.

Jam it back in, in the dark.