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Using plant-zombies seems de rigeur. Human or local goblin or orc cadavers could be one alternative, as well as local animals, like a bear or some boars. Along with the wolves, they might be sent out to patrol for local animals to kill and bring back to the lair to feed the plants / fungus gardens that sustain them all.
A plant/zombie-infested *owlbear* could instead be the biggest of the big bad threats he's managed to get under his sway.
But perhaps also he has taken over an underground warren created by a giant ant colony, that he has infested with his parasitic fungus and are now like cythnigots visually, but with the stats of a giant ant with the plant/zombie type modifications/immunities, and a special attack from the plant bits sprouting from it (like spores that cause disorientation, such as a sickened condition, but be careful not to make it too debilitating, or able to 'stack' to stunlock people, like Boggard's croaks or something, 'cause that can get frustrating when you are having a half-dozen of them attack, and having to roll all those annoying saving throws!).
Dozens of such parasitized ants could still exist, and be used to upkeep the tunnels, and patrol for intruders and fetch and carry things for the evil druid/necro. And, somewhere, deep at the heart of the tunnels, is the ant-queen... Perhaps she's also parasitized/infected. Perhaps she's *not*, and there's an entire section of the ant-warrens still inhabitated by uninfected ants, and they are fighting a losing war against the invading druid/necro and his fungus-infected ant-zombies! (The uninfected ants aren't smart enough to be allies, really, but they are the enemy of your enemy, so it's in your best interest not to kill too many of the uninfected ants, who are also fighting your enemy!)
Plant monsters which incorporate bones for cosmetic rather than mechanical effect could work. A psychepore firing bits of bone and some fungus leshies perhaps.
How low is low level, anyway?
Assassin vines, shambling mounds (an immobile one, if the APL is on the low side), and the yellow musk creeper.
Ghouls and ghasts are a little more interesting than skeletons and zombies. Could do bog mummies (and just your standard peat bog as a hazard; those things are terrifying). Maybe some "sentient spore clouds" that are incorporeal undead like shadows or wraiths.
You could have freshly dug tunnels that force characters to crawl on their stomachs, the odd animate root, hungry zombie or small monstrous spider making for a hectic and claustrophobic encounter.