Does the party have to be a surprise? Happy anticipation was always part of the fun for my dad. My mother began having family and friends parties for my dad on his 65th. We celebrated that one and every 5 years after that - including his 95th! He so enjoyed looking forward to the big day, as well as greeting people he hadn't seen since the last party! If at all possible, I strongly recommend letting you FIL in on the plans. :-)