Right, this is a game of obvious answers! So, the next obvious thing to do with the banana is?
To, give it to...
To the monkey, of course.
Well, that wasn't obvious. But the grinder left! You can now grab... err, borrow... the crank. The original monkey doesn't want to be the monkey anymore. He needs the crank back. Now, onwards with the obviousnesses!
So, the next obvious thing is?
To insert the crank to the jack, of course. And crank it good. The van with Goon and Jane is in the upper world now, away from the sewers! What a good friend are you.