do appositives only modify nouns or pronouns

You don't need a preposition because Pacific Beach is a fully defined location and "there" is fully defined by the name.

If you do not use a fully defined location then you need a preposition to help you define it. For instance when booking theatre tickets you might say "I would like to sit there, at the front.", this would not fully define your desired position so you would need the preposition. However if you said "I would like to sit there, seat B17" then B17 refers to a specific, fully defined position (a unique seat) so you do not need the preposition.