That adventure game I'm showing off at 4:35 is ECHOES OF THE END, a game I made! (not alone, jesus christ)

I absolutely love that 25-50% of your advice is “blatantly steal things you like from other sources.”

I have found layers to be very effective for my battle maps. When you add overlapping areas, suddenly players want to use their fireballs to blast open the ceiling so the monsters fall through it. It also adds more opportunity for making big hits feel big. Last session, my party fought an ancient blue dragon at a castle courtyard and its surrounding battlements and rooms. They chased the thing onto the roof of the castle, then, after burning its legendary resistances, landed a Hold Monster on it, and then the barbarian proceeded to blast the thing with four critical hits, and I described as the massive force of the conflict caused the barbarian and dragon to both break through the rooftop and the floors below, even through a room another party member was standing in, until they were all the way in the catacombs beneath the castle to continue the fight there. If I hadnt added layers to my map, this kind of shinnanigan wouldnt have been possible.

Oh so THIS is the video you were talking about.
Love the Tutorial idea early in the dungeon, I’ll be stealing that for sure.

Toys in Combat are also a great way to let lower level heroes fight a more powerful opponent.  Having an environmental healing effect can let you have a massive, epic combat at extremely low levels.  In a new professional game I am running, a death knight's chained specters that fueled his special powers turned into healing spirits when broken free from their chains.  A two hour epic battle with second level characters.

Barriers. Rivers, bars, brick walls, overturned tables, anything your players have to go around, over, or through. Or use as cover. Meaningful choices make fun combats.

Speaking of doors, I made a door mimic one time and my players have been afraid of doors ever since. I used that fear to make a dungeon with a hall of doors one after another and they all freaked out each time I unfogged the map and they saw the next door 10 feet away. Not one of them was a mimic, but the chest at the end of the hall was. Very fun. Permanent trauma.

3:24 "Come over here, I left you these siege weapons... oh those? No, it's fine, they're just red barrels."

"Click on their heads" games absolutely sent me. Ha!

i had a final fight planned against a hydra, in a sewer underneath a city. the extra mechanic was going to be a control console with levers that, when pulled, released scalding steam from various vents. but i knew i had to "teach" them this mechanic before the final room or else they might be confused and ignore it. so i had a few vents with associated levers earlier in the sewers, either already on and blocking their path or available as weapons against the giant spiders therein. it worked a treat. they got to the final room, saw the hydra, saw the levers, and made the fight memorable.

We had an entirely vertical fight which was a blast to run.

I cut a poster tube in half the long way, then used sticky tack to affix the minis, and twine to represent their 50 feet of rope, with binder clips attached to the ends so they could scale.

Then I had a table of things to roll. So there were dark mantles, giant spiders, flames, grey oozes, water floods, and down drafts.

Then, at the bottom of this vertical ascent was an umber hulk waiting, so there were some serious consequences for failing checks. Super fun session.

The Monster Hunter franchise will often put big unwieldy weapons with long cooldowns and very specific ranges on the board. The only challenge is timing, but there's basically a guarantee you get to hit a big button and watch number go BRRRR.

The word "Dragonator" being mentioned let's the franchise vets know:

A) Oh boy the big drill is back and I wanna press it

B) This boss is going to be MASSIVE

6:17 Also, if you've ever played the Hotline Miami games, regular doors can be a weapon.

Lair actions mentionned, yay

One of the great fights in the Pirates of the Caribbean movies takes place in a smithy or a mill, and there's a rumbling mill wheel, turning gears, and windows of opportunity to attack and moments of rest in the middle of the fight.  The Phantom Menace also does this in the three way fight between one Sith and two Jedi with energy doors that open and close. Both of these offer opportunities for both DEX saves (to avoid getting your clothes caught in a gear and getting pulled into a machine) and to avoid being partly fried by a closing energy door.  But they also offer the opportunity to ... shift your initiative number.

Let's say you've got a machine that's whirling around in the middle of the combat space, and there's an orc on the other side of the machine. The Orc figures out that the rhythm of the machine is set so that it's possible to attack on actions divisible by 4. So he could attack at 20, at 16, at 12, at 8, at 4, and then at 20 again. But then he pauses, knowing that the player whose turn is at 17 is going  to be frustrated by the whirling gears (or give the orc a +3 to AC or disadvantage to the attacker).  But the Orc holds his turn, and then shifts to a new number, and attacks through the machine that just frustrated the player. "Wait!" the player cries, "I had to fight through the machine with disadvantage! why does he get to make a regular attack!?"  And you, the GM, say, "well, he's studied the machine rhythm.  He's attuned to it, and fighting with the machine rather than against it."  And the players have to make an INSIGHT check or something to attune to the machine rhythm, or the Bard has to chant the rhythm of the machine to the fighter, before he can shift his own attack to be at initiative 12 rather than 15.  He loses the ability to attack earlier, now, but also loses the Disadvantage which the machine introduces.

Potions of [you ignore 100 hp worth of lava damage] means you can mess about with lava earlier. Placing important things in lava'd places will naturally bring drama, as their shields deplete.

On the topic of pitfalls and environmental traps. One of the most fun and rewarding combat encounters I ever ran was one I made for my level 4 party (5 players) where I threw about 20 kobolds at them. The cave they were in was split down the middle by a 15 foot wide, 700 foot deep ravine. One of my PCs uses a rifle, another has a Misty Step-like blink (homebrewed Dishonored class) so I felt comfortable putting enemies out of range to give them something unique to do and split the party's focus a bit. But the star of the show was the Ravine. One of my PCs; Borealis the Lamia is a Pugilist, and she specifically took the subclass that's all about grappling and keeping enemies locked up.

Borealis must have thrown 8 of those Kobolds into that ravine still alive over the course of the combat, each one accompanied by cheers from the other people at the table. She also argued that because she's a Lamia whose snake-half is 30 ft long, she should be able to just cross the ravine without a check, and honestly, that's just player ingenuity I felt like rewarding, yeah of course a 30 foot snake is gonna be able to cross a 15 foot gap without issue.

The jokes in this one as usual are spot on. Love the humor! The Dadi joke killed me.

What helped me the most is understanding distances. Place the siege weapon exactly at the maximum range a player can reach my dashing and jumping, this way it feels important to get there and there is a certain agency the players now have, either focus a player on it or suffer the consequences. If a party needs to put a member to a "job" to take care of, the fight feels like everyone is doing something important and thats just awesome.

Yes, yes, yes i just added exploding mushrooms and oily mushrooms as toys for my underdark campaign. I´m about to homebrew some mykonids that also have this feature when dieing... The underdark will be be a blast hehehe