You could combine the scavenger hunt and movie night...choose his favorite movie and have a scavenger hunt for things from the movie,then when you all get back...show the movie and have popcorn and movie food...our fav family movie is the western Tombstone...so we would look for things like playing cards, cowboy hats, trains, bandanas, shot glass, etc...then probably have brisket and baked beans...I may just have to do this myself, sounds like fun!! lol
Could also surprise him before the "real" movie by having a powerpoint "movie" of pictures of him over the past 50 years.