There are three apples visible in the Leisure Park trees which can be knocked down.
Frank cannot shoot the apples, Frank must throw something at both of them to get them down. If the apple doesn't fall out of the tree after Frank hit it once then it is probably stuck. These apples need to be hit at a good angle for them to fall to the ground. Aim slightly above the apple with the targeting item.
The two drink can boxes, found under the covered park shelter, are the ideal weapon to throw. The Covered park shelter with two drink cans.
In Infinity Mode, these apples are also present. However, an odd glitch often occurs where the apples near the North Plaza and the Food Court are permanently stuck in the tree, no matter how many times they are knocked. In addition, the third apple by the wall will not appear at all should this glitch occur. However, simply by eating the other food items found in the park (3 cookies and a snack located near the flower beds) or by storing them in Frank's inventory, the apples will all spawn correctly upon re-entering the area, and can all be knocked down with a single hit from any item.
Stand at the bottom of the middle section of the stairs that lead to North Plaza and face south. Walk south to a tree to the east.
This tree has a rock below it on the tree's south side.
Look up into the tree to find an apple on the west branch. Frank can throw the rock up into the tree to knock the apple down. But the Drink Cans are easier to throw.
Far west near brick wall
In the far west of the park, near the brick wall, look up into the tree with a rock near it to find yet another apple on one of the branches.
Along the path
Coming out of the Food Court there are 3 paved paths to take, take the middle. Go to near where that path meets the other and to the right should be 2 trees with rocks below them, the apple is in one of those.