Wanting to catch the Coaster Train in San Diego when I visit - I will either drive there from the Gaslamp District (2nd & Island) or walk to Convention Center and take light rail. It looks like Santa Fe Depot is the closest to get this train? If I drive, do you park at the depot or do you need to park elsewhere and pay for it? Does the parking work the same at all the depots? Also, what are the top attractions in San Diego that you have to pay for parking? Will help me decide if I want to use light rail. Thank you very much!

I would take the Trolley (light rail) as you suggested, or Uber/Lyft. It's not far and not worth the hassle of parking. I'm fairly sure there isn't much parking available.

As for attractions where you have to pay for parking: Seaworld, Safari Park, Midway Museum, Maritime Museum.

Parking in Balboa Park for the museums and the Zoo is free.

For the Midway Museum and the Embarcadero, it would be simplest and quickest to walk. You'd only take the Trolley one stop.