You don't say how you're arriving in SD, but if you're not coming on a Surfliner from LA you should certainly ride from SD to at least San Juan Capistrano if you have time, and if not, at least to Oceanside via either Amtrak or the Coaster. Of course the ocean is on the left side leaving SD. If you're interested in light rail, the original line is the Blue Line to San Ysidro, but to my mind is the least interesting. A more varied trip is alongside the Amtrak/Coaster (ex-ATSF) route to Old Town (the station building there is fairly new but constructed in retro style to look as though it's been there forever); then through Mission Valley at least to Grossmont (or to El Cajon or Santee if you have time), returning via the other line (I think now called the Yellow) to Imperial & 12th, then via the Harborfront Line back to the Amtrak station. Between Old Town and Grossmont the line parallels a freeway which is some distance below it farther down the hillside. The rail line tunnels into the mountain to stop at a subway station under a university campus, then reemerges to parallel the freeway again. On the return there is a horseshoe curve just west of 32nd Avenue for an abrupt climb out of the valley. Another curiosity is a point just east of El Cajon where the line diverges to form a single-track line on the surface, with a grade crossing, for freight trains, while the trolley crosses on an overhead bridge to avoid having the frequent trolley trips block the highway. There is also a heritage PCC service operating on the downtown loop during certain hours; the car is not from San Diego but carries the traditional SD PCC livery.
Also (always assuming you have time) the diesel light-rail line from Oceanside to Escondido, over an ex-ATSF branch, is worth riding; since local freight operations take place at night, it has standard GCOR signals throughout, as well as platform extensions at every station which are lowered for light-rail operations during the day and raised out of the way after the end of service before freights can be admitted to the line. There is also a dual, opposed point derail just beyond the platform at Escondido which is set, when the light rail is operating, to derail any eastward movement from the main track to the yard beyond as well as any runaway westward movement from the yard to the main track. (It's the only dual derail I've ever seen or heard of.) This line also has a spot where the light rail leaves the original alignment to serve a college campus, crossing a freeway twice on overhead bridges to do so, while the original single-track line continues on the original alignment until the light-rail track rejoins it.