In his classic tome, Making Shapely Fiction, author Jerome Stern describes one (of many) fiction archetypes called Blue Moon stories. Blue Moon stories, according to Stern, "appeal to our deepest selves. We enter the world of magic, myth, and dream...our sleeping world, our childhood tales, our religious beliefs are full of happenings whose reality is not of this earth." These types of stories can make great suspense, spooky, or fantasy novels.
These types of stories aren't just for grown-ups either. Classic blue moon stories from childhood often leave a lasting impression on our world view, inform our dreams, and inspire us in later life. While it's tempting to go crazy with your imagination in these types of stories, they can end up with cliché ideas that readers will reject, and it can be hard to be fresh and new.
If readers like the world you're creating, though, chances are the readers will follow you into it. Their 'willing suspension of disbelief" will get them through the door, but you have to keep them there. How? By having the storyteller go along for the ride with the reader and admit their limitations (I'm simply reporting what was told to me long ago by my grandfather, who heard it from his grandfather, and so on).
Building the odd or mysterious events into the narrative early and having the narrator also experience moments of disbelief about the strange happenings is an effective tool. It helps the reader relate to the narrator and they'll be more open to magical and spiritual concepts.
You can and should use magical and mystical elements in your story, but keep the rules consistent. If you are creating a galaxy where all beings are wisps of smoke, don't have them lounging in a chair. If you are working through time travel scenarios, make sure you lay out the rules early, and don't cheat the reader by breaking them at the last minute.
If you are mixing in normal people, places, or things into a story with magical elements, make sure the reality details add up. If your character that sees dead people has to pick up dry cleaning, follow the rules of our world and make sure they have their ticket. Be particular about plausibility. If magical or spiritual elements are going to resonate with readers, the world around them needs to be believable. You don't want your readers to get sidetracked figuring out why "that would never happen" when it's something simple and real and we all already know the rules.
Save the special unbelievable elements for the fantasy aspects of the story...the magic, the spirits, the inner voice that calls all of us to believe in blue moons.