Have things happen around them. Let them interact with those things as they will.
A merchant shows up with a cart of goods. The merchant happens to be a kobold selling shrunken heads, or live mice with their tails tied to a stick, or herbal charms to ward off spirits. if the party doesn't react, the locals do. Maybe someone harasses the merchant, maybe someone assault them and chases them out of town. Doesn't matter if the party interacts because maybe later when they are on the road the find the merchant dead and his cart plundered.
Then have a fight break out, maybe someone in the party is misidentified as an adulterer, or a bandit. The party can plead their innocence, fight or flee.
It doesn't matter what the party does per se. It matter that the world is alive around them, and that they can chose to interact with it or not. BUT, if/when they don't, things still happen. They don't help the adolescent who said their grandmother was possessed, then a demon or a hag shows up. Or something bad happens. The world is happening around them, and if the party doesn't get involved, then bad happens more often than good. Evil grows.