There could be a character's odd behavior or a mysterious symbol mentioned earlier that hints at the big event at the end.
Maybe a minor conflict or a seemingly insignificant conversation early on actually sets the stage for the main climax. For example, a passing comment about a hidden treasure could lead to a big treasure hunt at the peak of the story.
Often, it could be a recurring dream or a prophecy that foreshadows the climax. Like a character constantly having a nightmare about a specific place, which turns out to be the location of the final showdown.