"[C]onsider the nicest, cutest, sappiest, and sweetest quests in the game; nothing sad or depressing but sweet and light."
Personally my favorite "nice" quest in the game is Orphans Like Cookies Too! in Stormwind City. It's one of the random daily cooking quests and tasks you with searching the various inns and general stores throughout the city to gather bags of confectioners' sugar to help Robby Flay finish a batch of cookies for the city's orphans. It's short, simple, and sweet as a sugar cookie. =P
One of the reasons I like the quest (outside the obvious fact that you're baking cookies for orphans. Cookies. For orphans.) is that it's all the more meaningful if you've paid attention to WarCraft's lore. The cookies aren't just being baked out of the blue, they're being baked at the behest of King Varian Wrynn, who was himself orphaned at the end of the First War. Despite becoming one of the most powerful figures in the Alliance, it seems he's still able to spare some thought towards making the days of his city's most misfortunate a little bit brighter. A little sugar goes a long way, indeed.
PS: I totally didn't realize that the Confectioners' Sugar could be purchased from certain vendors around the city until I looked at the WoWhead comments while writing this post. I thought they were only obtained from looting the Sack of Confectioner's [sic] Sugar objects at the inns and stores. /facepalm