In "City of Danger" right in front of you there is a monkey on a ledge who will give you a wrench in exchange for a banana. In front of him there is a crate of bananas, walk up to it and pick one up with. Then when your blue target is on the monkey, press again to toss it to him. He will then throw the wrench. Don't pick it up, and wait a few seconds, and he will pull out another one. Exchange wrench for banana again. If you toss him a banana before he pulls out the wrench, he won't be interested in trading, and it will just land on the ledge and you won't get credit for it. Repeat process until unlocking.