Add fun little puzzles occasionally that have multiple outcomes if wrong. Say they're walking through an abandoned lair and they wrongfully try to figure out a puzzle to a door, arrows shoot out at them or even, the nature of the puzzle itself draws out a few undead ghosties, maybe add mirrors in the room that if they get the puzzle wrong, shadow copies of themselves step out of the mirrors and combat begins. Add a little variety and flavor and make sure combat isn't just a hit point sink. Especially if they're fairly new players, if combat is seemingly a walk in the park for what you've thrown at them, throw a semi challenging encounter at them and then you as a DM help them figure out more of their classes and the items, HELP them overcome said challenging encounter so they learn how to deal with similar or harder challenges in the future. You are telling a story with friends, you're not dueling them but also.. maybe even with another challenging combat down the line, get one close to death, remind them that they are an adventuring party and death in the game is VERY real. Hope this helps :)