Generally I try to tell the truth, but sometimes it's not a good idea. Sometimes you can TELL that if you did tell about the cache the moment you left the it would get muggled.
I too carry a pretty substantial camera and make excuses for picture taking. Dick carries a covered clipboard and looks official, he has a number of OSHA orange T-shirts with his Local 3 Union logo on them. They generally think that we are out on some official errand.
I say be creative! Think of it as a creative writing class. I personally loved a number of them, Snipe hunting being one of my favorites.
My personal favorite would be:
"If I tellyou, then I'll have to kill you. Do you REALLY want to know?"