Here’s a basic formula that you can use to construct an encounter:

Challenge (combat, social, puzzle or other) + unique element (memorable NPC, fighting on a rope bridge, etc.) + a way to advance even if the party fails (although perhaps with penalties) = a successful encounter.

And here’s a brief example encounter:

A band of orcs (the challenge, combat) + the fight takes place in small boats on a rushing river (the unique element) + the orcs will take the PCs prisoner if the party loses (a way to advance despite failure, but with consequences).

By including only a single challenge and single unique element that defines the encounter, you’re keeping prep time and on-the-fly bookkeeping to a minimum, and keeping the focus on what makes the encounter fun (everything is within your players’ flashlight beam).

By making sure the PCs can move on even if they fail (partially or completely), you’re preventing the encounter from being a roadblock to keeping the adventure moving. And when you’re bleary-eyed from prepping for your next session, having a formula to glance at might just come in handy.

There are lots of other ways to build encounters, but this simple approach will work for most groups and most RPGs.