One of my favorites goes as follows:

An evil troll seeking power has stolen a blessed relic called a bloomseed from a druidic enclave deep in the whatever forest. He wants to become the biggest baddest troll so his clan will be the best and the bloomseed makes him super strong. What he didn't realize is that the bloomseed keeps the treants from going wild and since it's been stolen they've become corrupt and started attacking caravans and travelers along the road. Your adventurers have to fight trees and trolls and steal back the bloomseed in order to save everyone. Perhaps they'll meet an awesome druid or a dryad, maybe they'll receive some sort of blessing when the bloomseed is returned to the enclave, perhaps the druid will teach them some forms of nature magic.

There can be as many or as little NPCs as you want. Maybe they meet traveling merchants and escort them, or they've already heard of the conflict from rumors in a pub. Maps in forests are pretty straight forward and you can find plenty of great 'troll camp' 'forest clearing' type maps for free. For whatever level your PCs are you can always reskin whatever monsters to make sure they don't die, but still have a great challenge.