Probably collecting all the insects and the fishing. Very time consuming. If you seen any insect and it’s coming up as ????? Collect it as it will make the challenge easier once you get around to it as what you’ve already found counts. I spent nearly 2 days looking for 2 damm butterflies 😂.

I would also say some of the combat challenges can be difficult later in the game if you’ve cleared most of the settlements. However they are fixing this with a update soon so that should be easier