My first visit to a flight deck was when I was 5 years or so. It was on a LH
Boeing 735 from CGN
and I remember it quite good as it was pretty funny: The pilot asked me if a would like to sit on his knees, take the yoke and "fly a curve".
Of course nothing would have happened as I guess that the autopilot was in command, but I was too shy or afraid at that time and said no. Then he typed something like "Welcome aboard and have fun in Munich" in the FMC and printed it out for me. That was quite nice.
But the best ever experience was in 1997 or 98. That was also on LH
(this time an A320) and while boarding I asked the F/A if I was allowed to have a look at the flight deck. She went to ask the pilot and retuned saying "You can go right now". I was happy that I was allowed but also a bit dosappointed because being in the cockpit while in flight seemed to be more fun than on the ground.
Well, but when I came into the flight deck the First Officer had gotten out of his seat, waiting for me, and then flapped down the jumpseat for me and showed me how to buckle up and where to grab an oxygen mask in case of emergency. Yes, I could barely believe, but I was allowed to stay in the flight deck for the whole flight!
I think I was the happiest boy on earth that day...