When he rises from R'lyeh - by some magic mojo by the good guys, he gets dimension dumped into several alternate universes.
He plops into the ocean and sinks to the floor. Can he be successful there or just stay on the bottom and eat shrimp. In the scenario, he can either be detected on the way in or manage a stealth landing.
a. Mikdemia or Kelewan. You can have the Academy and Pug working together on Kelewan (before he left it). On Mikdemia, you can have the Valherhu active or not.
b. A major Culture world's ocean
c. Star Trek Pacific during TNG
d. DBZ
e. Star Wars - Coruscant
f. I thought about Dune - but a desert world seems unfair
g. Earth from the Galactic Patrol Lensmen books
I don't want these to be a stomp but tough worlds outside of the normal Lovecraftian antagonists.
If he just gets vaporized, I apologize. In the Frank Belknap Long books that were in the approved continuity, Cthulhu did take an H-bomb and just went back to sleep.