This task doesn't result in a steam achievement, but it does give you a cosmetic reward. It is necessary for the "Complete All Items on Your To-Do List" achievement, though, if you're trying to 100%.

All of the balls for this quest are located in the same area as Big Potato. Normal tennis balls also don't count, so don't worry about collecting them! You only need to find the rainbow tennis balls.

Steer the tennis balls into the hole Big Potato is digging for them to count.

The first ball is located in a nest in the nearby tree. Climb the vines on the greenhouse, jump on the branch, and destroy the nest.


(The nest is already destroyed in these screenshots but you get the gist.)

The second ball is located on a balcony above Big Potato. Climb the vines on the greenhouse again, and then jump on the air conditioner.


Smack it off the railing toward the ground. If it falls into the balcony, there's slots in the bottom of the railway you can smack it through.

The third ball is located underneath a bucket right next to Big Potato. Smack it to knock it over.