If you're reading my blog since the beginning you already know what I think about dungeons.
If not, you can find it here.
After more than 20 years of gaming and DMing here's a list of the top ten things to put in a dungeon.
Note you should not place all of them, but you can pick what you need from time to time.
Of course this is only a partial list: dungeons are marvelous places where you can find virtually anything; if you have other ideas just let me know and I'll expand the list.
1) At least one hidden room, very difficult to find, with a special monster or treasure, or maybe both.
2) A locked door with a trap who will strike upon all the PCs.
3) A NPC who offers to help and guide the party but in fact he's gonna to be a traitor.
4) An enemy who will bargain his life for some special knowledge (some info about the dungeon, or about magic items or else).
5) A NPC party raiding the dungeon for the same purposes of PCs and who will struggling with them.
6) Some humanoids, using some of these tactics.
7) An area with particular rules (wild magic, dead magic, difficult terrains..).
8) Some vengeaful spirits (or other undead creatures) of previous unlucky explorers.
9) A connection with the Underdark, with Drow coming from it.
10) The party caught between 2 groups of monsters (or 2 factions of one group) fighting among themselves.