On our recent trip I stayed at one of the hotels near San Diego Convention Center. while was very much a business oriented area of the town I did get the feel that there were many things that one could do while in San Diego for free. Old town San Diego has been restored and is a very beautiful place to walk.
San Diego also has many natural parks that are beautiful as well. Because San Diego is in the southern region of California there is a very pleasing climates to be enjoyed so any type of free outdoor activity could be a very useful idea if trying to save money while in the area.