Hug bug: a bug (or insect of any kind really) that will wander from street to street, go and hug the person on the street with the lowest mood percentage--bringing them up enough to give them the next level (or two levels) of smiley face on their mood counter, and then move on to the next street. Other people on the street could hug the bug themselves for a more modest mood boost.
Perhaps there could be one per area, traveling in a randomized pattern that makes it go to each street in its area, but not always in the same circuit. Each area's love bug could match the decor theme. Groddle Meadow, for example, could have a ladybug with hearts on its shell instead of spots. You might have a giant firefly in firebog areas, a mole cricket in Ilmenskie, a bright scarab beetle in Uralia, that kind of thing.
There could be badges for giving and receiving bug hugs.
The real purpose, of course, is because it's cute and fun! But mood boosts and badges too, for those people for whom cute and fun isn't enough.