I am creating a dungeon and the main baddy will be a ghost. However, the ghost isn't going to be straight out of the Monster Manuel, I'm going to really jazz the ghost up to make it spooky and unexpected. The ghost is going to haunt the entire place, and pop up in a variety of way such as possession, telekinesis, cold areas, etc. Additionally it will have some different vulnerabilities like salt. What I could use some help with are brainstorming ideas for ways ghosts haunt things (ie. possession, telekinesis, etc.), and ways ghosts have been destroyed or made vulnerable (such as salt). In other words, what folk lore or movie/television/book ways of Haunting and escaping can you think of? Any idea is great, and I'm going to try to incorporate them into my dungeon ghost monster.

I remember from the show Supernatural the most common way to defeat a ghost is to find the original corpse and cleanse it with fire. Alternatively, the ghost might be haunting a specific object, which needs to be found and burned instead.

My DM just has a ghost taking possession of inanimate objects. We’d destroy one then it jumped to another. We fought it in, basically, a cafeteria, and I found myself attacking random tables to cut off places for him to go. 
Then, of course, there were the odd suits of armor and other things he possessed in other spots in the dungeon. We eventually found his body and took him to bury him properly to finally be rid of him. 

There was a 2nd Edition Ravenloft guide on Ghosts.  Van Richten's Guide to Ghosts maybe?  https://www.dmsguild.com/product/17494/RR5-Van-Richtens-Guide-to-Ghosts-2e?src=hottest_filtered&filters=0_0_0_45753_0_0_0_0

It has been awhile since I read it and my memory isn't perfect, but it seems that guide was one of the better ravenloft monster guides.  It expanded on ghosts significantly: powers, categories (type I - IV, if I recall correctly, where each type was significantly more powerful), hauntings (now to see if I can find my hardcopy buried in my old rpg book shelves).

Some thoughts that address the OP's original question:

• One way to defeat:  For the most powerful ghosts there may be only one way to release them.  Fulfilling their task or purpose, reminding them of their death, reuniting them with their locket.  the side effects are that these kinds of ghosts are largely immune to any other types of damage or harm.  They might be chased away, banished, or temporarily subdued, but they always return until their "task" is achieved.
• I had a ghost possess a door, the door in and out of the dungeon.  He was the gatekeeper, bound to seal the dungeon away from the outside world.  In his madness he would let no one and nothing leave, including the PC's as they tried to escape the flooding dungeon.  They had to get creative to find a way thru the possessed door.
  • (He also could only be "defeated" by being sacrificed with the sacrificial dagger that was used to sacrifice him as a mortal originally.)
• In one story a fellow DM ran years ago: The Ghost could only be "defeated" by returning her locket to her, and only by placing it around her neck, not as simple a task as you might think.  (We had to find the locket, figure out the locket was hers, then figure out it belonged around her neck.  There were plenty of clues, paintings, inscriptions, books, lore, etc.)  This particular ghost wasn't the big bad evil guy of the story, mostly something to be avoided, but it was necessary to defeat her in order to defeat the BBEG.
• I had a ghost in another adventure that could be made vulnerable by showing her image in her hand mirror.  She could be defeated other ways, but using the hand mirror was the "storybook/roleplay" way of doing so.

For all the words I don't feel like I added much of use to the conversation.  Hopefully you find something useful in this.

I ran a haunted house story a few months ago.  One of my characters was a lesser noble and in return for a favor, the local Duke gave him this cool abandoned manor in town.  Of course, it was haunted.  I took the "Death House" from CoS and repurposed it to my liking.  Some atmospheric things the players enjoyed:

- There was a spectral party in the ballroom.  Spirits were dancing to music and ignored the party members.  Later, the music stopped and the spirits vanished.  But then in the large mirrored walls, the characters saw their reflections were now slowly dancing around the room as if in a trance.

- They had a painting done of their party and it was hanging over the fireplace.  When they first arrived to explore the house the painting was normal.  Later, when they re-entered that room, the painting had changed.  The noble was still standing front and center, smiling... but now all the other characters were bound up like BDSM nightmares and the noble had them all on leashes.

- At one point the players tried to escape through an upstairs window.  There were commoners on the street and they had a shouting conversation with them... eventually all the commoners stopped moving and stood there as if frozen... a shadowy figure emerged and one by one killed the commoners in the road before tipping the dagger to the party and then approaching and entering the front door [I stole this bit from another forum... don't recall who to credit].  Players who tried to go out one window "fell" back in via another window.

- And all along, the ghost was having private telepathic conversations with the noble player.  The ghost was offering to become a warlock patron for him if he just agreed to deliver one of the other characters as a sacrifice.

- On the upstairs landing, a suit of armor had their attention and they pre-emptively destroyed it.  Of course it was then that the real threat, the mimic-chandelier sprung to action and started pulling characters up off the ground.

The final fight with the ghost was anti-climactic.  But its the atmosphere that counts in a scary story.  They were convinced they were up against some kind of arch-devil by the time they got through the house.  Make their imaginations turn your ghost into something truly memorable.  Build up that terror!