Non-fantasy adventure novel with good romance, featuring adult protagonists

I've scoured other recs on here but almost everything seems to be fantasy and/or YA. Maybe what I'm asking is impossible, but here goes:

A well-written book for adults, featuring adults in the real world going on some kind of adventure and falling in love throughout the course of it. Their relationship should be based on mutual respect and well-earned trust that build as the book progresses. Happy Ending. Bonus points for some spicy scenes.

Thanks in advance!

ETA: I don't mind a supernatural element or two, as long as the story takes place in the real world. I love ghost stories, for example.