18. What you need: nothing
Just outside the entrance to Milk Road, there's a large patch of tall grass. Walk around it until you find butterflies flying around a particular spot. Go to the butterflies and you'll fall in a hole with a large Pea Hat inside. Kill the Pea Hat to get a piece of heart. (This must be done during the day. If you go at night, the Pea Hat will be invincible).
19. What you need: Guilded Sword (optional)
Just outside the entrance to the Mountain Village, there are a bunch of snowy mushroom like things (like the one you got the Kamaro Mask from). Underneath one of them is a hole with two Dodongos. Kill the Dodongos to get a piece of heart (The Guilded Sword makes this MUCH easier).
20. What you need: Goron Mask (optional), Zora Mask
Just outside the entrance to Great Bay Coast is a large rock. Punch it away with Goron Link (or use a bomb) and jump in the hole. Use Zora Link to kill the Deku Baba's. Then knock down the bee hives up above. One of them has a piece of heart. Just dive down with Zora Link to retrieve it.
21. What you need: Goron Mask, Goron's Lullabye
Outside each entrance of Clock Town is a hole. Each of these holes has 1 large Gossip stone and 3 small ones. Go into each of these holes and play the Goron's Lullabye for each. After all 4 are red, you'll get a piece of heart.
22. What you need: Mask of Truth, 10 Rupees
Go to the Dog Race Track in the back of Romani Ranch. Put on the Mask of Truth and bet on the dog who begins his thoughts with "Ruff." Bet on this dog and you're guaranteed to win. If you win over 150 rupees, you'll get a piece of heart.