In Lethal Lava Land, the easiest (and funniest) way to collect the 100 coins is to enter the "Hot Foot It into the Volcano" star, then move forward from where you start until you get to the yellow ! box. Get the shell from the box and surf around the level (somewhat carefully) to get tons of coins. If very careful, you can get 100 coins and not even have to go in the volcano.