Something has awakened in the forest
Legend says that that the forest outside of town was once a place where every plant had intelligence, speech, and a will of its own. A few people would insist that had seen a shrub speak to a tree in an audible language. But these stories were always shrugged off as folklore.
Take your PCs through 3 stages of the forest. In the first, they meet Awakened Shrubs and Awakened Trees. With some investigation, the shrubs and trees will tell them of a great disturbance that has drawn energy into the forest. The shrubs and trees will tell them that the forest itself has taken sides. Some creatures think they deserve this new power. But the lowly shrubs and trees prefer the usual state of things.
SECRET: They were always awakened. They just keep their mouths shut.
In the second stage, the PCs are attacked by a pack of baboons, a swarm of ravens, and at night, a swarm of bats. In each attack, they realize that the creatures seem affected by some kind of madness. They do not have to kill the creatures but the creatures will fight to kill. When they have survived these swarms, a Giant Boar or Giant Constrictor Snake crashes through the brush to attack them.
In the third stage, treants can point them to a cave where a Green Hag — defended by a coterie of Lizardfolk (vary the number depending on your party size) — is using terrifying elemental magic to reshape the forest. Throw in plenty of forest creatures who may view the party skeptically (as if they’re in league with the hag) as they travel through the woods.
NOTE: the hag is carrying a Wand of Web which she will use to ensnare PCs, allowing the Lizardfolk to attack them more easily.
(Grab a name for the hag if you need one.)
If you have a party of level 6+, you may want to add a Shambling Mound or Earth Elemental — created by the hag — to attack them when they find the hag. The hag may either escape to leave a thread for further adventures. Some Lizardfolk may leave with her while others defend the retreat.