K.I.
I always thought it would be fun to take him somewhere fun (Napa, Vermont...) for his 40th. As for a party, I personally would stay away from all of the over-the-hill garbage and black balloons, these are depressing and not festive. I had a Friend who sent out invitations with a picture of her husband at 2 years old, naked in the shower. They were pretty funny. Good luck, have fun!