Rescuing all three Jigglypuff in the Cave track. For saving them, they'll perform for you nearby the exit. Even just rescuing one of them will get you adorable results, though of course it's best with all three.
Zapdos restoring electricity to the Tunnel might count, especially since the background music becomes less ominous.
Toss some food to those two Charmander in the Volcano and watch as they call over their four friends to share the spoils. Your reward is (if you can get them to move to the right positions) a shot of all six of them marching in unison to the Poké Flute.
Pikachu watching Diglett pop out of the ground is really cute, as the latter looks absolutely delighted every time he sees his friend and will ignore any Pokemon food thrown nearby in favor of continuing to play.