We tried to find "Balboa Park" using our phone's navigation on our first day in San Diego... it led us to a ditch near the zoo. We didn't get a chance to look again until late in the afternoon on our last day (by that time all the museums that we wanted to visit had closed). It was still fun to just walk around and there is tons to see besides the plethora of museums (architecture, gardens, trails, fitness courts, art demonstrations, outdoor theatres, etc.).
Start early and go to the park's Visitor's Center first (1549 El Prado, San Diego, CA 92101). Balboapark.org is also a good place to get more information for planning your visit.