Funky Monkey Yogurt
Make sure you clear out your inventory as much as possible.
This is where you'll want to have the Pencil Eraser handy also. If Paula had it before she was kidnapped you can get it from Escargo Express.
Once you've freed up some space in your inventory, inside the Drug Store pick up a Picnic lunch and Skip sandwich.
Monkey Trading Co.
What comes next is a series of trades that you'll make in exchange for the Monkey's moving out of the way of different tunnel entrances.
Use MAP 2 in order to plan our your route and find where you can find the items needed to make it through the caves.
Just past the monkey that asks for the Picnic Lunch, there is a Pizza. Go to the very next room and give that Pizza to the monkey Monkanna. There is ANOTHER Pizza in the rooms past Monkanna. If you give the first Pizza to the wrong monkey, you'll have to go all the way back out of the caves and order a pizza from Mach Pizza by calling on the pay phone.