I like the "real" dungeons of old games.

- the dungeon is closed until you find the keys and solve the puzzles to get out (no teleport back, no weapon shop on every third level.

- you have to search for food to avoid starvation.

- dungeons have to be dark - make torches or cast spells for light.

- dungeons become really interesting if you can get poisoned, cursed, ill and so on …

- puzzles, hidden doors and trapdoors, levers and traps and teleports are must haves, too

in short: just take the old dungeon master as a blueprint