I just ran a solo-adventure for my player a 2nd level Monk. The sewer was REALLY small: 100 ft long, ending in a cave-in, with 2 more tunnels on each side ending in cave-ins after 20 to 40 feet.
I added a few "slime puddles" on the ground to make combat interesting: Balance check DC 12 or fall prone. Or you could just Jump over them

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The 2nd level Monk barely survived the encounters I placed:
3 Fire Beetles (EL 1) "Lairing" at the end of the first tunnel. My player killed them no problem.
3 Dire Rats (EL 1) After the battle the Fire Beetles, the Dire rats were alerted by the presence of the Monk. One Dire Rat went to scout ahead and was killed by my player, after wounding her once. She then engaged the other 2 Dire rats, but was almost killed, so I had to "cheat" a little.
I figured the Baric feeds on the Dire rats, Fire Beetles, and what not, so the Baric came out to "save" my player. The Baric killed one Dire rat, while my player killed the other. The Baric then proceeded to attack my player, who was able to kill the Baric after a few rounds.
1 Baric (EL 1) This is a six-legged rat that can be found on the Creature Catalog site hosted by Enworld!
Hope this helps!