It's hard to make adventures that work for the overpowered Leeroy Jenkinses of the world.
It's axiomatic: the higher level the characters, the tougher it is to make challenging adventures for them. Here are a few tips and tricks for creating excellent adventures for characters above 15th level.
Take them outside their comfort zone.
Gary Gygax was famous for making high-level adventures that challenged his players in unusual ways. He ran a level of a dungeon based on Alice in Wonderland logic. Another "dungeon" was a crashed spaceship. He sent his PCs into the Wild West, with dynamite and six-shooters. His infamous Tomb of Horrors featured insta-death traps that killed at a touch.
This is where you shift play to other dimensions and planes, to other times and settings. That can mean literal time travel, BTW, in which the PCs have to be careful not to erase themselves with a paradox! How about adventures underwater, or in space, or inside a volcano? Try a new kind of adventure ... a bank heist, an espionage mission, sabotage behind enemy lines, conquering a kingdom, defending against that demon invasion and closing a hellmouth, winning a poisoned pie-eating contest, climbing the tallest mountain, and trying other new things your heroes have not done before.
Power them down.
The big problem with making high level modules is that the characters are very powerful! One solution is to decrease their effective power. You have to use these methods sparingly, or else the fun decreases. Here are some ways to challenge characters by surprising them with a variety of power-downs.
- Declare schools of magic off limits. I once threw my PCs into a demiplane in which all divinations and dimensional magic failed. No summons, teleports, or blinking; portable holes and bags of holding stopped working; summons failed altogether.
- Use the environment against them. I once put my high-level group into a burning hellscape that inflicted 10d6 fire damage per round in hurricane-strength winds. Their immunities and resistances made this trivial, of course, but I set them against fireproof treants who were using sunder attacks against all their gear. Once an item was sundered, it broke into pieces. The pieces, no longer touching the former owner and lacking fire resistance, burned to ash and were dispersed by the wind. Another time I used the cave-in rules to bury them under rubble, pinned with no save allowed, just a DC25 Str check to push yourself free. Then I sent in ghosts and spectres to attack them! Muahaha!
- A clever DM I know once challenged her players by cursing them switched bodies! Players had to swap character sheets and play their own personalities running other players' characters.
- Another very reasonable way to power down a high level party is to let each PC bring just one friend along. If a PC has an animal companion, an eidolon, a paladin's mount, a familiar, a cohort from the leadership feat, a figurine of wondrous power, and a homemade golem, well, sorry, only one is allowed. You might just outlaw cohorts and constructs entirely, allowing only friends derived from class features.
- Finally, the most boring but maybe the most effective way to cope with high level PCs is to make like the Fed and slow inflation -- power inflation, that is. You can stop excesses before power rises too high. Simply do not put any items with bonuses of +3 or better into the world. Make sure that no magic item with a gold piece value of over 10,000 gp is ever found. Similarly, strictly limit the amount of gold that you distribute, to ensure that the crafters in the party do not have much gold for crafting. Also limit inflation by making sure that there are very few high level NPC wizards, no spellbooks with powerful spells, and only the two new spells per level that wizards pick up as a class power.
- In fact, make sure you limit your splat book use ... going beyond the core rules into optional and extra books is a sure-fire way to get overpowered fast. Approve each splat book rule on a case-by-case basis, with an understanding that the permission to use it can be modified or revoked if the rule proves overpowered. (BTW, here's my rant on why splat books are bad!)
However, there is a major drawback to the "take stuff away" strategy: players hate it! They worked hard to earn all the things they can do, and taking some away seems unfair. The stress comes from losing capabilities AND from not understanding why. A good way to mitigate the stress is to explain what the limits are and why they are happening -- even if the characters might not know why, make sure the players do, or else you, the gamemaster, come off as arbitrary and rules-breaking.
That being the case, how can you challenge high level PCs without taking stuff away?
Surprise them with new monsters and magic items.
Novelty goes a long way to spice up any game, even high-power ones. Give your players new challenges with new things to encounter.
From armies of kobolds to mobs of trolls to lairs of liches to the Tarrasque itself, do not be afraid to throw unusual monsters and combinations of monsters at your PCs. Reskin that Tarrasque as Godzilla or some other Kaiju. Sure, let your players have some easy victories from time to time -- they are awesome, after all. But also try pitting them against things that they must flee! Throw the odd elder dragon or demon lord at them once in a while. Interesting adversaries make for interesting adventures. Even lower-power foes can be interesting if they are clever, using the right items, traps and environmental advantages, for example. Keep in mind that your PCs are specialized against certain monsters: rangers against favored enemies, clerics against undead, paladins against evil, etc. Give these specialists opportunities to outshine the other PCs from time to time.
One danger here is in making NPC foes that are too hard for you to run properly. Creating a party of 17th level NPC enemies -- wizard, cleric, barbarian, and ranger, say -- is a thankless task. Generally, only run lots of leveled casters if you consider yourself a varsity-level DM ... those complicated foes are just too painful to run well when you have a bunch of them to track.
Your players might enjoy a very different kind of foe with some friendly player-vs-player gaming. Imagine a tournament allowing only nonlethal damage, with the PCs up against other NPCs and each other! Offer metamagic rods of merciful spell, saps, and the standard rules for inflicting nonlethal damage.
You can also mix it up with magic items. Do not be afraid to include custom items, cursed items, and even artifacts. They're high level; let them grapple with that kind of treasure. And giving the party a mixed-blessing artifact lets you open the door to an artifact-destruction quest!
I'd be remiss if I did not mention the Web serial novel "Worth the Candle." This story has tons of great ideas for encounters, items, and house rules that will challenge your high level characters, for sure!