Super Mario 64 on N64 - The Green Cap (Metal Cap)

The green cap is in Course 6. To find it, head there and go all the way down to Dorrie's cave. Then climb onto Dorrie and make him take you to the big gray door on one side of the cavern. Go through that door and jump into the dark liquid to reach the green cap level. Try not to fall into the water. On one of the edges, you should find 2 red coins; grab them and keep going.

At the very end of the path, grab the 2 red coins and then jump on the green switch. Next, break the green block, take the cap, rush into the water and quickly grab the 2 red coins at the foot of the pillar.

Keep going around the pillar to find the 2 missing red coins, and grab the star while you're still Metal Mario so you don't get swept away by the current.