Definitely do a sunset dinner at the Fishermans on the pier.Actually any time of day is good to go there.
Several restaurants on Del Mar,Beachfire is good.Same owners have the Irons in the Fire restaurant at the municipal golf course.We go there for family breakfast get togethers,very good.
About two blocks south of Del Mar at the corner of El Camino Real is a locals taco shop called La Tiendita.Their breakfast plates are huge,if you like a mexican breakfast.,and cheap,like 4.50.
If you enjoy walking, the San Clemente beach trail is great.Runs for several miles from North beach down to the state park. If you walk further past that on the beach,you will walk by President Nixons western white house,and the epic surf beach called Trestles is just south of that.
Back at North beach,at the train depot,a bike rental place just opened up a couple weeks ago.You can ride portions of the beach trail(need to walk the bridge and near the pier) or head the other way,and ride to Dana Point Harbor.
Dana Point Harbor ,as mentioned ,is the home of Capt Daves Dolphin Safari.
At the harbor also check out the Ocean Institute,a learning facility for school age kids.Tidepools are right behind the building.