The Lava Cavern heart piece is being guarded by the alligator. In order to move around in the cavern you need your iron boots. Jump off a ledge that is near a magnetic field below and immediately put on your iron boots. I can't remember exactly where the chest is for the heart, just that is was guarded by the alligator. The other heart piece I think has to do with the Goron that is standing near the bridge opening. If I remember correctly, you need a bucket of springwater to throw on him to refresh him; once done he gives you a heart.