Thinking about the sorts of riddles and puzzles I'd like to include in my megadungeon. Gygax (and maybe Arneson as well?) was fond of math and logic puzzles, and of course puns. I'd likely use some of those.
But my background in English means I'll also use lots of language based (non-pun) riddles, and allusions to literary works. For example, kennings from Beowulf or the Prose Edda of Snorri Sturlusson. References to Shakespeare or the Romantic Poets. Trivia from Geoffrey of Monmouth, Edith Hamilton or Sir Thomas Malory.
I'm envisioning 'puzzles' where, for example, there's a pit or shaft descending deep into the earth. Above it, carved in stone, is the phrase "Aegeus saw the black sails." If you just take a leap of faith and dive in (as Aegeus did when he saw Theseus's ship return with black sails), a featherfall effect will take you safely to the bottom, where there's some swank treasure or a secret passage past a nasty encounter or something like that.
Or maybe a map leads to a secret door, and has a kenning like "The warriors crossed the whale-road, carrying bright war brands." Draw your sword and splash some salt water on the door to get it to open.
Might be a bit more interesting than the typical "3 gallon and 5 gallon jugs, place 4 gallons of water on the scale to open the door/defuse the bomb" type puzzles.