Well, you could start using the Mythic rules, either for the PCs to make things more interesting to them, or for the DM to make monsters any scarier. It's Paizo's way to make lower level characters interesting or adding onto characters to make them more Epic feeling, though it's very different than the 3x Epic stuff.
Unfortunately, I don't know any Epic stuff for Pathfinder. I know I'm currently writing an Adventure Path, but I'm only planning for it to go to 20. Otherwise, pretty much all of Paizo's APs end well before 20, except for Wrath of the Righteous [You could possibly take some of the Foes from the last book. With the high levels + Mythic, they would probably be somewhat challenging for your group]. Unfortunately, Paizo doesn't seem to like high level stuff. My Council of Thieves character ended the campaign at level 11 and my Legacy of Fire character was 13.
I though some of the later Bestiaries have like Old Ones, like Cthulhu and Hastur and stuff. Not sure which books exactly those are in, but supposedly they're above 20 and could be a challenge for your group.