Most San Diego neighborhoods either took their main street for granted or lost it outright. The South Bay spent two decades rebuilding one on purpose, a walkable commercial stretch with an identity of its own, and the street fair is the day it shows you the work.
Third Avenue closes to traffic completely. Vendors set up along the pavement, several stages run at once, and the food is the tell — this is one of the most diverse cities in California and the fair does not sand any of that down into generic festival fare. People come up from across the southern county for it, and the crowd looks like the census, which is rarer at a San Diego event than it ought to be.
A full free day in Chula Vista, with dates posted at thirdavenuechula.com. Go with somebody who has only ever driven through on the way to the border.