The San Luis Rey River Trail stretches all the way from the woodlands of Guajome Park in east Oceanside to the Pacific, poking out conveniently in between the Oceanside Pier and Harbor. It's a favorite ride of mine, being mostly separated from car traffic other than one street crossing and it's relatively flat so if you're loaded up with cargo for the beach there are no huge hills to climb.
If you're heading east to west a quick detour at Douglas Drive will take you by Old Mission San Luis Rey. There's a lot of history tucked away while exploring the beautiful grounds.
A little further down the trail at Foussat St. (the only place you'll have to look out for cars on the ride) you'll pass the skatepark. If you hear "WHOOOOO" look up - chances are you'll see some skydivers coming in for a landing the airport just south of the path.
A few more minutes of pedaling will take you to the end of the trail which is kind of the beginning of the adventure. There's a ton of stuff to do from here. If you turn right it will take you to Oceanside Beach and the Harbor. The harbor has all of the aquatic activities you'd expect of a beach town. Personally I enjoy watching the sea lions. If you're looking for a snack there are quite a few restaurants to choose from.
If you turn left at the end of the trail you'll head into downtown Oceanside and the Pier. When I do this ride I stop into the Stone Company Store on Tremont which has a great selection of their beers on tap.