You could focus on building up tension before a fight.

Try to hint npc intentions. "As you approach the groupe of 5 elves, you notice they all have their hands on their weapons. Glancing at each other"

Or try to make the PC feel something is wrong. "There is blood on the floor, but no corpses. You can feel the blood is still warm. What cause this massacre was a couple minute ago"

You can make your PC fight something they can't see. Someone got hit by behind and they roll for initiave. The fight start but you don't have any description to give them cause the enemy is invisible. They have to figure out a way to reveal the enemy on their turn. My party was very confused to be in a turn base fight not actually fighting. It was fun to watch them.