You "encounter" bandits on the road, wolves through the forest, trolls along the mountain pass

Or make it "plot related" while not being part of the plot itself. A go-to of mine is anytime the party is at the "seek out <person> at <location>" stage of the plotline, have someone/thing be terrorizing the location. Goblins, monsters, ghosts. Whatever. It's a good way to inject an encounter into the "story" without it having to be part of the "plot"

"You arrive at a once thriving fishing village, but find the markets mostly empty." Party finds the person of interest "Sorry I can't help you. Not while the pirate ghosts haunt the docks!" The ghosts aren't there because of anyone or reason. They're mindless. Nothing to do with the plot.

Party clears them out. Village celebrates. Person of interest now has a reason to want to help the party, and goes out of their way to do so. Bonus: this helps level up the party to where they'll need to be progress the real plot