Can you give us a bit of a clue as to what he likes/hobbies etc and how much you were planning on spending. Then some of us might be able to give you some ideas.
This may sound silly, but it was very much appreciated by my parents on their golden wedding. A newspaper printed on the day of his birth !! Several sites available, just Google them. Mine came with a certificate showing what things cost, who else was born that day etc.
Depending what his interests are and how much you want to spend, I bought my Dad & stepmum for their joint 70th birthday, tickets for The Rat Pack show with a pre-show dinner included. Total cost 100 quid and they both said it was the best night's entertainment they've ever had! And for the pre-show dinner they were treated like royalty. Well worth spending every penny just to see the looks on their faces when they opened the tickets!! whatever you get your Dad - he will love it coz it's from you!!
I've just posted to the question about the 30th birthday about my friends site www.saysomethingspecial.co.uk you can do an online classified announcement simular to what you see in the back of papers its only a little gift but your Dad could keep the print out forever.