I've made quite a few cockpit visits, but sadly none since 2001.
In the late 60s and 70s I was a frequent 'schoolboy' traveller to France. Dad worked for BEA and organised the tickets; took me to the airport and walked me through the Terminal to the gate, and then often disappeared to speak with the crew ... quite often, on boarding, was met with "ah, here he is ... this way please", and ended-up being in the cockpit for the whole trip. In those days it was mostly Tridents, and occasional AF '727s.
It definitely gave me a better appreciation of what goes on during the flight.
In the late 70s, coming back from hols in Malta ... staff ticket, the whole family got 'bumped' due to overbooking. They eventually find 3 seats for us ... one cabin-crew seat 'down the back', and two in the cockpit of a TriStar. Came back all the way in the cockpit wearing a headset and chatting with the flight crew.
Sadly, the grey-matter was not up to the job, so never managed to make the grade.
In the mid 90s I went to SW France on Hols ... BA 737 out of Gatwick. En-route I asked for a look in the cockpit. Got chatting with the crew, asking about the radios and avionics ("wow, that's a new question, never been asked about that before! ..."). They asked why I was going to Bordeaux and I told them I was taking some photos ... got invited to stay in the cockpit for the rest of the flight on the basis that I took some photos for the crew as we landed and taxied. PNF folded down a small 'dicky-seat' and handed me his camera!
Took a few photos on approach, on touchdown, and as we taxied; can't remember how many pics, probably about a dozen? I hope they came out okay, as pre-digital.
A fantastic memory, sadly no longer available.