Dude, you -need- to get the Immoral's Handbook Epic Bestiary Vol I. I has EXACTLY what you're asking for. It answers exactly this question... what DO you use to challenge high level characters? It doesn't stop at 100 either...
Upper Krust suggests that you can have meaningful (combat) interactions with any other being as little as half, or as great as four times your own CR. This describes the transition all the way from Nemisis to Cannon Fodder. For level 100, we're looking at CR 50 Cannon Fodder, or CR 400 Nemesis. Specifically, The IH lists an Anakim at CR 50, and an Orichalcum Gargant at CR 365, with 25 published monsters inbetween. Each monster comes with at least three plot hooks to build around each monster, so you could have an entire campaign based around these things. These plot hooks range from Low Epic (21 to 40th level) to High Cosmic (621+). 100th level is listed as "High Epic" (one below Low Cosmic) and most of the monsters in the book have a High Epic challenge listed.
An example plot hook: "A pact of four Akaliches converge to cast the Epic Level Spell 'Army of Darkness' which can slay and animate entire nations. Once conquered, this will be their throne world, the first of many death planes under their command."
Sounds like a perfect game for a one-shot. The Akalich template increases a target's CR by 53, and of course, you can always start with a base creature that's already epic. The example Akalich in the book is only CR 94 (ECL 121), but you can easily bump that up a few. And Akaliches can create guardians for themselves. The Epic Level Spell "Schism" only has a spellcraft DC of 100, WELL within these Akalich's reach, and not only does it seperate the divinity from gods, creating the fearsome Quintessence Elementals, (CR 52 for severing greater gods, perfect for the cannon fodder range) but when then killing these elementals, it can unleash the Terrifying Unelementals, which deal permanent damage (same CR, but still scary).
And that's just for starters. This book makes it clear that Amazingly Insane PC Level Gaming is not impossible, does not have to be a joke, and is entirely playable. I -highly- reccomend it. If Akaliches aren't your style, a Cogent (World Flayer) might work out for you (CR 101). Or perhaps you want to run an evil campaign... you could set your sites on the King of Heaven (CR 126) or even one of the seven Seraphim, once cosmic champions, now purveyors of balance (they found out what happens when you start killing gods) at the frightening CR 231... your level 100 characters will be able to face the Seraphim... so long as they don't have a level adjustment of more than 12, since any creature with fewer than 88HD die instantly simply by comming within 500 feet of it. If angel killing isn't their style, how about facing Golems made from Star Matter? A Gargantuon Orichalcum Golem is about at the upper limit of what these players will be able to face (with fast healing 100, DR 100/--, 4600 hp, and immunity to magic just for starters... it's offensive abilities include a free action ray usable once every other round for 300d6 damage and 18 points of ability damage to three different stats, and even if you kill it, it explodes dealing massive damage with a DC100 fort save to simply avoid being vaporized)...
And then, to finish it off, you can throw stuff at them that they litterally have no chance of handling. Just to demonstrate that level 100 is nothing compared to what's in store for them later on...