It's been a long time since I've played the vanilla game as I usually play randomizer, so I don't remember if they are heart pieces, but the locations I always forget are:
- Mimic Cave (mirror back to light world when you get outside of Turtle Rock halfway through)
- Breakable rocks to the west of sanctuary (largely because I forget to come back after getting the boots)
- Lumberjack cave (dash into the tree after beating Ahganim)