> This got me thinking about Commodore 64 games. One of my favourite "hidden parts" in a Commodore 64 game is in "Wizard" by Progressive Peripherals & Software.
> In the level "Ladder land", the topmost gold ball is guarded by an invisible enemy. Get a key to gain some spells and shoot at the ball to kill the invisible enemy. Then go to the top of a ladder and jump to the platform with the ball.
> The ball is in fact magic - and its magic effect is to recreate itself, in the exact same place. So you end up getting the same gold ball over and over again, for as long as you like, amassing points while you wait. I have managed to score over ten million points and get about 30 extra lives this way.

Another fun thing to try:
In "Gateway to Apshai", whenever you find treasure, use the "Locate trap" command. If the treasure is trapped, walk over it again and again while the trap symbol is still flashing. You'll collect the same treasure multiple times, amassing more points. You'll also set the trap off, though.
This only works for treasure. For items of actual use (weapons, armour, potions, spells) the item will become a "ghost item" - it looks like it's there but it isn't.