Im sure there are many in San Diego, CA ..
like In&out burgers and such.
However I think one of the most popular San Diegan foods are Carne Asada Fries
May look totally weird but its basically french fries, carne asada, guacamole, sour cream and we add lots and lots of hot sauce :-).
So not healthy but soooo good. Def one of the foods that we have people try when they are visiting from other parts of the states.
you can also get it in a burrito known as a California Burrito
of course there are MANY other weird foods and things you can find but I would say these are the most popular and known by the locals.