Instructions
Play through the game until you have a conversation with Katrina Badfellow. She'll show you the exit to the cave high up on the wall. Walk forward and press the "Space" bar when you're near the picks to grab them.
Press "Space" again when you're up against the cave wall to climb it with the picks. Go to the right along the wall until you reach a small ledge with a shovel on it. Press "Space" to pick up the shovel.
Climb to the left along the wall until you're just below the exit. Press "T" to switch your active item to the shovel. Press "Space" and you'll place the shovel into a nook in the cave wall. Shaggy will then automatically use the shovel as a springboard to reach the exit. A brief conversation will follow and Scooby-Doo will automatically follow you through the exit.