If you know how to get the dot from the secret chamber in the original Atari 2600 Adventure game in the Black Castle, the same trick applies here, although you don't need two other objects to make the wall disappear to enter the room and see the programmer credits, just carry the dot as it is into the room for an updated surprise.
Beat the game in all levels (Easy through Expert) on game number 3 to unlock the Classic Q1 mode, which has the original Adventure graphics.
I'm not exactly sure how this works, as it seems strange that you have to beat all levels (Easy through Expert) to unlock the Quest 3 secret, as I beat the game on Quest 2 on only three out of four levels in order to unlock Quest 2.5, but if that doesn't work I assume beating Quest 2 on all levels will unlock 2.5.
EDIT: I think I hit a bug for the above, since all the other modes to unlock you have to beat all the games for it; I had beaten three out of the four games and exited the program before the cinematic ending finished, and suddenly 2.5 was unlocked, even though I hadn't beaten all four games yet. That or I had beaten the game on Expert first, one of the above two, I'm just not going to reinstall and start over the game from scratch to be sure, it's too infuriating!
To unlock Quest H, beat all levels (Easy through Expert) on Quest 2.5.