This has been said before, but I'll put my spin on it, for the sake of covering all aspects of adventure design.
When making an adventure, I try to put in an equal amount of role playing opportunites, combat and problem solving.
Role playing is any encounter where the characters get to interact with an NPC in an interesting way, or that gives them a chance to explore their character. Negotiating with a greedy merchant. Deciding what to do with freed slaves, deciding whether to keep treasure that they know is stolen.
Combat is fairly self-explanatory, but always be answering the Dramatic Question of why the characters are fighting, and what the stakes are. Also, every fight can benefit from some kind of complication. Burning barrels of pitch, shifting tiles, swinging platforms, fog, darkness and/or hazards.
Problem solving can be as simple as figuring out how to ford a dangerous river, or as involved as a magic puzzle trap in a mad wizard's tower. It can also be logistical in nature. Figuring out how to transport a large hoard of treasure.
I don't always get the ratio at a perfect 3/3, but I do try to shoot for that ratio, and if an encounter overlaps with two or three types, all the better.