Wow, 50 - that is a big one!
Something like that should be celebrated with a sentimental gift in my opinion, so your dad can look back at it as time goes on and enjoy it. Doesn't mean it can't be complemented by a monetary gift too though.
Have you thought of getting his friends and family together and writing your wishes down? Maybe gather some photos too and put everything in a book format. I've used this site called
CelebrateAHero.com, which actually let me invite anybody to chime in and add their wishes / photos. They also printed, bound, and mailed me the book.
As far as monetary gifts, maybe consider a nice watch that will last your dad a lifetime. Maybe a Tag or the like?