We stayed in Oceanside only because our son in law is a Marine stationed at Camp Pendleton close by. We were pleasantly surprised to find a quaint walkable down town area with a very nice beach, pier, harbor, all accessible by walking or biking paths.
There are many small and good cafes, bistros, and diners down town, at the harbor, and including a good diner on the end of the enormous pier. The beach is large and watersports, especially fishing and surfing are popular. The weather was fantastic in May with cool weather suitable for long walks or bike rides.