@Shay I'm not sure if these image will get you to the cave entrance, but this is how I try to orient myself to reach the cave.
Start at the smaller fishing area in National Park (the one without the waterfall) and head in the direction with your back to the water, along the rocky hills on your left.
It's not a marked or clear path from the water to this point, but you'll eventually see two large boulders on each side.
(These boulders aren't far from the grilling area.)
Walk between them and keep heading in the same general direction, along the rocky hills on your left.
Eventually, you'll pass a slope on your left. This means you're getting very close.
At this point, even if you direct your sim to walk straight foward, they will make a curved path around the trees on the right.
Very soon, you will see the brambled entrance to the cave ahead of you.
Hope this helps, Shay. Good luck!