I've run a lot of sewer bashes over the years, the most recent in a short lived Freeport game and ended up making a Word document of potential things the PCs could run into.
It included:
Giant Crocodiles
Normal Crocodiles
Giant Toads
Rat Swarms
Giant Rats
Giant Rat Monster Thing
Gelatinous Cubes & other Oozes
Carrion Crawlers
Otugh (Of Course)
Chuul
Zombies
Ghouls (Used as friendly NPCs, out looking for some dead guys to eat. Follow PCs around long enough, and they find something)
Wights
Serpent Men (Yuan Ti, although the Freeport version)
Giant Octopuses
Thieves Guild Members up to no good
Toshers
Abolythes
Environmental Hazards:
Sewer Gas
Crumbling Walkways
Sudden Sinkholes
Sewage Whirlpools
Flash Flood of Sewage due to unblocked pipes/heavy rains or Ogres doing their business
Wave of broken wood, waste, and detritus from the flood above - very dangerous.
Extra Slippery bits of floor
Stuff dripping down from above
Fatbergs
Dead cats, dogs and other animals - possibly in advanced stages of decomposition. For extra giggles, have them bloated with gas so when a PC stands on one, or hits it, or falls on it, it explodes in a burst of yuck.
Corn
Tosheroons (Not a hazard, but a cool way of giving out treasure)
In a previous campaign the PCs ran into a Drow Arachnecromancer who had animated spider exoskeletons, which then lurked under the sewage. Being hollow, they'd fill up with the sewage, which would then spray everywhere as the PCs fought them. The poor Rogue got pinned under one, stabbed it from underneath, and then got covered in effluent, as a jet of the stuff came flooding out.
Describing the sheer disgustingness of being in the sewers helps as well - especially what happens if Characters fall in. The fear of drowning in effluent is far worse than the fear of drowning in mere water.
The Terry Pratchett book 'Dodger' has a wonderful amount of easily usable background for 'fun' set in the sewers.
