1Terminal
The best place at Santa Monica to be up close and personal with the action is the airport administration building. This is rather new and offers superb photo opportunities. It has a small viewing area at the right side of the building that is somewhat elevated and offering picnic tables and benches. There is even a speaker broadcasting the local ATC frequencies. You are so close to the runway that not much more than 200-250 mm is needed for a Cessna Citation. So with some heavier equipment even smaller aircraft - if that is more to your liking - are within reach. After about 3 pm the sun transits the runway centreline, so best photography is in the morning. Finally most of the aircraft parked at the other side can be read from the viewing area.
At the left side of the building there is an Asian (Japanese?) restaurant with terrace, one level higher.
Both locations can be accessed from outside the building. There is ample free parking in front of the administration building.
2Sports fields
This is the most remote area from spot 1 and you need to get here for spotting the aircraft parked around this location. These cannot be read from spot 1 due to the distance (heatwaves!) and aircraft being parked in front of others.
Spot 2 can be reached from spot 1 via the Airport Avenue, 23rd Street and Ocean Park Boulevard. It is at the end of 25th Street, next to some baseball fields at Clover Park. Parking is easy here in the residential area. The airport authorities have provided some telescopes here for more viewing pleasure. Photography is difficult due to the fence and back light from the sun.
3Supermarine FBO
Most of the Supermarine FBO ramp can be seen from spot 1, but some aircraft are hidden behind buildings and hangars. No worries, just drive to spot 3 past the FBO building where the missing aircraft can be identified. Photography is difficult here because of the position of the sun and probably nervous security staff. So a hit an run is the best you can do here.