Love Potion

Since the effect started nudging NPC happiness upward it became genuinely useful. Lob one into a cluster of townsfolk and their prices soften for the duration, which is worth doing right before a large purchase from a merchant who dislikes where he lives. It cannot push anyone past the happiness ceiling, and it does nothing for a homeless NPC, so it patches a mildly grumpy town rather than fixing a badly planned one.

The burst hits everything in range, thrower included, so expect to trail hearts around for a while. You cannot cancel it yourself and the Nurse charges to clear it, which is the only reason the game files it as a debuff. Nothing bad actually happens to you.

Princess Fish sell for considerably more than the potions they turn into, so brewing a stack purely to cash out is a loss. Catch them, keep a handful of potions for happiness work, and sell the rest as fish.