Zelda 1 - All heart containers in the first quest of The Legend Of Zelda on NES
Overworld Map - Quest 1 1 of The Legend Of Zelda

There are only 5 heart containers in the overworld, but they're not easy to find. An old man often gives you a choice between a container and a potion. You'll have to choose the heart container as potions can be bought elsewhere.

Heart container 1 (Bomb in M3)

Place a bomb down on the right side of the rock to open a door.

Heart container 2 (Bomb in L8)

Place a bomb to the left of the Northern(upward) path.

Heart container 3 (Candle in H5)

Burn the fifth tree from right to reveal stairs.

Heart container 4 (Raft in P4 up to P3)

Use your raft in P4 to gain fourth heart container in P3.

Heart container 5 (Ladder in P6)

Use ladder to obtain the last heart container.