I have a (what I feel is) pretty cool abandoned mansion "dungeon" designed.  It's based on some old Celtic castles. There are all of the rooms you'd expect to find in the 1700s and 1800s, solar, cabinets, budoirs, great hall, library, study, garden room, dining halls, servants quarters, galleries, etc.  My hope is that the party spends more than one session exploring this mansion (it's 30+ rooms) but I want them to be excited about searching every room, so I want there to be rewards cooler than just a sack of gp in every desk drawer.

What kinds of things would be cool or interesting to find in these rooms?

There are so many possibilities for loot in castles and mansions; Golden/silver candle sticks, fine clothes, encrusted wall sconces, fancy animal furs (can be used for armor or winterizing gear), animated armor carrying +X weapons/shields, portraits, plethora of different jewels of varying values, musical instruments, mirrors out the wazoo, ingredients and materials for crafting. The types of books ranging from general notations all the way to fully filled spell books containing high end spells, spell components and find inks, scrolls which are empty or previous imbued with magic, forgery documents and wax seals, perhaps the deed to the castle/land itself or a share in its rights of Status; religious symbols or statues of the god/gods they worshiped, if any, gear for a wagon or horses, items to replenish an adventuring pack (burgler, explorer, scholar, etc.). The list goes on and on and that could just be all in one room depending on the size of the estate, and that is mostly just materially valuable items.

Like Oboe posted, asking yourself questions about the previous owner can REALLY fill out a "dungeon" like this!

If the house has a history of violence or betrayal, think about the kind of undead or fey creatures that might be attracted to such an environment. In terms of architecture, remember that the kitchen would be placed at a corner of the house so that the smoke can easily be vented to the outside.

A sex dungeon.

Have the players find that the cult of the black goat has been using the inner workings for their discussing parties. You could get creative with it. They might be wanting to summon something nasty (Not that kind of nasty you sicko.) into the world.

Maybe ancient golems or ghosts?

I definitely want to hear there will be ghosts in there. Maybe not all of them hostile.

I’d want to find wall safes or boxes hidden behind tapestries or portraits. Different rooms can have different spell books in them obviously used by the previous occupants of that area. The kitchen can have Purify Food and Drink, Haste, Locate Object, and a spell scroll for Create Food and Water shoved in a drawer and labeled “In case of unexpected guests.” The bedroom could have Charm Person, Alarm, Sleep. The office/study would have Comprehend Languages, Illusory Script, Enhance Ability, Unseen Servant. The greenhouse could have Detect Poison and Disease, and Plant Growth, as wells as a Staff of Flowers, and a decanter of endless water. 

As far as flavorful treasure, You can find coin collections, rare books, gold bird cage, music boxes that play classical tunes, chess sets with premium material pieces, masquerade masks, silverware, fine china. Wine or other fine liquor.

Common magic items are Pots of awakening. Chest of Preserving, Heward’s Handy Spice Pouch, Cloak of Many Fashions, Orb of Time, Talking Doll, Veteran’s cane.

And I also picture this locale as a good place to find a Figurine of Wonderous Power.

Mystery and clues! 

There could be an overarching narrative about the former occupants that players slowly uncover as they explore. Finding clues and hints is what will lead them to the solution of the mystery which could lead perhaps to uncovering a secret treasure or putting an undead spirit to rest. There should be secret doors and hidden compartments, and clues in journal entries and portraits. Think of it as a large puzzle with clues and hints possibly in every room. Hopefully this entices players to explore the whole place.

There was an old module like this. Chateau d’Amberville (Castle Amber). I remember it being really fun. 
Two things I remember most were a painting you could get sucked into and a big feast. I don’t remember the details on the painting, just that it was cool. The feast was really neat. IIRC, you sat down and were served a bunch of courses. It was all magic, no waiters or ghosts, just food started appearing. Each course, you had to decide if you were going to eat or not. After you ate, there was a saving throw (one for each course you ate), if you passed the save, you got a bonus, failed, a minus. Bonuses were things like you gain or lose 3 hit points, permanently (As in your hit point max changes). I realize the math in this edition is much more finely calibrated, but little things like that I don’t think would break anything. You could do things like that, and maybe give things like bonuses, or minuses,  to skill checks, let them gain or lose some gold, or throw in some temporary effects (bless or bane for the rest of their time in the chateau, for example). 
There was no explanation of what was happening, so make the first course low stakes, so they can figure out what happening. You don’t tell them what the save is for before they choose each course, so there’s a bit of blind luck/faith involved. 

Insane first wives/firstborn sons.

Oh, and a creepy doll's house, especially if it replicates the mansion itself.

Quote from faze4guru >>

I have a (what I feel is) pretty cool abandoned mansion "dungeon" designed.  It's based on some old Celtic castles. There are all of the rooms you'd expect to find in the 1700s and 1800s, solar, cabinets, budoirs, great hall, library, study, garden room, dining halls, servants quarters, galleries, etc.  My hope is that the party spends more than one session exploring this mansion (it's 30+ rooms) but I want them to be excited about searching every room, so I want there to be rewards cooler than just a sack of gp in every desk drawer.

What kinds of things would be cool or interesting to find in these rooms?

 Full leather gimp suit in a room with a rack (the torture kind), some whips, manacles, etc. maybe one manacles is really dimensional shackles..