Mini-Games: Part 1
Okay, Bill Shakespeare didn't actually say that, but that's his loss and not yours.
Mini-games add a wonderful variety and change of pace for your adventures. What are mini-games? They're any kind of puzzle, combat or roleplaying encounter, or activity that is self-contained and might not follow the game's traditional mechanics.
A puzzle that requires varying Skill Checks to hack into a computer network is not a mini-game. It falls squarely within the default mechanics. But a puzzle that requires stacking your dice (instead of rolling them) and then solving a riddle is a mini-game. It has introduced a new mechanic to the players, but that mechanic won't persist beyond the boundaries of that mini-game.
Mini-games allow you to catch your players' attention and to get them thinking with variety -- instead of always thinking, "Well, I'll roll some d6's again and try to kill this thing." And if a mini-game is challenging enough, without being infuriating, it alone can carry an evening's play session.
First up: puzzles and riddles. These are an easy starting point and are already used frequently.
Try to find something relevant to the setting. If players are exploring an ancient tomb, they should encounter a different flavor of riddle than if they were performing a bank heist.
While players are trying to solve the riddle, let them use Skill Checks to gain advantages and hints. Plan these hints ahead of playing so that you're not wasting time trying to devise the perfect hint.
Know your audience. If your playing group doesn't like riddles, don't keep throwing detail-picky riddles at them. If they do like riddles, try expanding out to a series of scavenger hunt events where the riddles lead them into combat and roleplaying encounters -- use different events as rewards for solving or failing the riddles.
Go do an Escape Room (in real life, not a video game). Afterward, write down all of the puzzles that you remember. Congratulations! You now have a handful of puzzles that were memorable and could be solved without a PhD. Populate a cave, crashed spaceship, corporate heist, etc. with any of these puzzles to gate how players move through the encounter.
Start with puzzle systems that already exist, like Sudoku, nonograms, and lights-out games. Save yourself the hassle of trying to create new mechanics and solutions. There are tons of these puzzles already, most people have some familiarity with them, and they can be easily incorporated (i.e. hacking a computer, reading an arcane scroll, crafting some complex item).
Pay attention to your players' abilities. A platforming puzzle that requires an agile player to jump between ledges, while a strong player pulls a lever and then an insightful player is reading the instructions from a foreign text allows each player to contribute.
Fabricate items that increase and decrease stats. We have a (currently unpublished) one-shot adventure where the players have to transport 6 orbs of varying colors up a mountain. Each colored orb either increases or decreases a specific stat, so the players are forced to evaluate when best to transport beneficial or harmful orbs in order to get all of the orbs up the mountain.
Start small, and keep a list of the puzzles and riddles that worked well. Whenever you’re playing video games, pay attention to the side quests, mini-games, and puzzles that might come up. How can those be incorporated into a tabletop RPG setting?