Have you read the adventure creation advice in the Epic Level Handbook? There are some good ideas there. And monsters.
You know about advancing existing monsters, right?
I recommend
Design Secrets: High-Level Adventures by Monte Cook for anyone who is running higher level adventures. It's a different game at high levels and you need to be ready for it. And this is the article that had it all click for me in my head, and
I learned how to stop worrying and love the bomb.
The fact is that 20th level adventurers can accomplish certain adventures in a day or two that would take a 15th level party a month to do, and would be impossible for a 10th level party to do. Each "dungeon room" for a 20th level party could have been the entire "dungeon" for a lower level party. And each relevant adventure site could be spread out enormous distances between planes, but it's just a
Gate and a
Teleport away. They only need to know where to go. So just chart out your adventures as a series of sites where each site only becomes relevant when the previous site reveals clues and information that indicates the next site is important. If there is a big bad evil guy driving the plot, keep his identity secret that only become revealed at the next to the last site. Until then, the party is only fighting his minions and stoogies. When creating adventures for a high level party, I use my experience in writing adventures for a superhero roleplaying game.
And if you are feeling bad about not being able to challenge your party, just understand how much fun it is for the players to finally play some buttkicking superheroes that can finally blast through game elements that they had to struggle with in the past. Now
they are the big bad powerful guys in the world.
What has been going on in the campaign lately, plotwise?