Food + Golden Hearts.

[Triple J]

Here's two suggestions-in-one:
Food could be collected as occasional drops, or from certain Treasure Chests. Consuming any food from your inventory could restore HP, or even have other, special effects!
Golden Hearts could be collected to globally increase your maximum HP by 1, regardless of any equipment already set to increase it. In the Level Designer, your HP cap should reset after exiting the level, to prevent any breaking of other levels.
Certain measures should be taken so that Golden Heart spam doesn't crash the game, methinks.