You Know You can Return to a Dungeon

This is something that newer players and GM's overlook and the modern "adventure path" play style discourages,simply,

Just because you went through a dungeon doesn't mean you can't revisit it. Its quite possible to go back to search for more loot, clean it up later, claim it for your own or do any number of other things. Its even plausible for different players or even the same players to play different characters visiting where there last PC's went.

Simply note the changes and play.

A word of advice though, if the same players plan to return with different characters I recomend the old if trite "journal" routine. This way any metagaming that goes on (and its hard to prevent it) is fair and fits better into the game world. This feels more organic and I think it leads to a better game.

Secret Doors and Puzzles in D&D

Big Purple has a discussion of this going but other than the reappearance of Old Geezer (the young fella at Gygax's table and a RPG.net legend) its not an exciting discussion.

My impression of secret doors and puzzle traps and all that is simple

These serve much the same purpose as unlockable content and puzzles do in computer games. Its a reward/another thing to do if you play it over again (with another group or by returning to the area later) or if you are clever about it.