My wife's happiest celeb encounter was Patrick Macnee at a luggage carousel. Very frail but very charming.
We also met Tim Brooke Taylor in Madrid airport. They kept reallocating our plane so we played "One gate to the tune of another." He was a lot of fun, his wife looked like she'd heard it all before.
In both cases the celebs led the conversation, I generally leave them alone, they need their own space too.
When I was a kid living in the Philippines I met Michael Caine and Lance Percival at the Manila Yacht Club.
They were in the country shooting a film called "Too Late the Hero".