I have two adventurers doing a requested one-shot at level 20. I have a Caviler and either Psion or Wizard. I'm a bit worried about the Caviler under preforming and will likely adjust monster stats to have some higher spell resistance/saves to fight the caster.
Anyways, what are some of the more fun end-game creatures that you like to see?
two man eating flying hippopotamoose....Thats always fun
I would reccomend against using CR 20 or higher creatures, since your players are going to be getting dropped into a level 20 character without having taken the time to learn the nuances that come with that particular build, and therefore won't be playing these characters to their full potential. Also, remember that you can always slap class levels and templates on any creature to try and bring it up to snuff. It's much easier to make weak monsters buff than the other way around.
Throw some iconic high level creatures at them, Treants, Giants, Demons, maybe a dragon even. Just make sure they are creatures you can effectively run without the game tirning into a sloggy borefest.
Getting the caster to use buffs, debuffs and crowd control would be a good idea. Encourage this with the mention of lots of potential fights. A melee character can go all day as long with a few wands of cure light wounds, a caster needs to be more conservative.
Most importantly do not have the bad guys use save or lose effects, if one party member fails, especially against some type of dominate effect that's probably game over.
Big beefy bad guys. A Dragon would be appropriate, high spell resistance.
Is the cavalier going to have a flying mount? Their charges can be pretty devastating.
I would probably use some encounters that have 3 CR 16-19 critters. One to die in round 1 due to nova, one to die in round 2 when it gets full attacked by horse & rider, and one to die in round 3 after doing some cool things to the PCs.