Right before the Checkpoint is a long "hole" of sand, you need to jump in the sand and let it carry you down all the way, you'll end up in an underground level, hit the "P" which will turn the blocks into coins, you'l fall through a BUNCH of coins...probably get a 1up from it and reach the bottom, head to the right through the green pipe. once you emerge jump over the green pipe in front of you! don't go down it yet!. you'll see in the middle of the room another "P", hit it, platforms will appear that will allow you to reach the 1st coin. now you can go back and jump in the first Green pipe I told you not to.
there you go!