Visits to the flight deck.
Sadly, for the 99.999% of individuals who pose absolutely no danger to the safety of an airliner, visits to the flight deck are a thing of the past. Any notion that the flight deck crew would be happy to sit and chat to a spotter after shut-down is the stuff of cloud-cuckoo land. After probably their 4th or sixth sector of the day, they'll just want to get home like you.
In 1980 (as an airliner mad 13-y-o), I wrote to the Chief Captain of Dan-Air asking if I could travel in the jump seat on one of their flights. It was all arranged in a 'Jim'll Fix It' style, and later I travelled on a 727 to Stuttgart and back.
Such a treat would be ABSOLUTELY inconceivable today due to all the rules, regulations, restrictions, and liabilties which now govern not just aviation but every area of our lives.
If you really want to be in the cockpit during a flight, your best chance is to go to your local aerodrome and pay for a trial lesson in a Cessna.
Please don't shoot the messenger!