Clambake!
Swimming contests, races, archery contests... and plenty of lobster, stuffed clams, clam chowder, corn on the cob, watermelon, and other goodies!
For the more sedentary, a storytelling competition (as opposed to the standard bardic contest) and maybe even a pie-eating contest if we can figure out a way to score it.
And this could be anywhere. Waterdeep is on the shore, Ford Keep is on a river, and Sigil... well, surely an appropriate spot could be found or created somewhere near the Bazaar.