Oceanside Harbor, or The Jetties, is a popular surfing area, especially in summer south swell season. Fishermen can try their luck off the end of a jetty (beware of high surf and high tides), or surf cast on the south side by the San Luis Rey River mouth. You can also catch a sport fishing boat out of Oceanside Harbor or launch your own at the boat ramp.
Oceanside City Beach, know to locals as The Strand, is an ideal spot for children to swim or ride the whitewater on boogie boards. There are a number of small parks along the boardwalk with restrooms, picnic tables, barbecues, and an excellent playground by the pier. Other beach-related attractions include the nearby California Surf Museum and seasonal surf and volleyball competitions.
Buccaneer Beach is south of the pier where the beach narrows and the road dips down to sea level. It's a popular spot for kids to practice their surfing with easy access and lifeguard supervision close at hand.

PARKING - The parking for Oceanside beaches is a mix of free, paid, metered and private (for slip holders in the marina), and much of it is time-limited, so check the signs carefully before choosing a spot. There is ample free parking on the residential streets and towards the train tracks, along with metered parking and self-pay lots.

BATHROOMS & SHOWERS - Plenty of bathrooms and showers at Oceanside City Beach and Buccaneer Beach

LIFEGUARDS - Oceanside beaches are well staffed with lifeguards on duty throughout the year

POPULAR ACTIVITIES :
Surfing, boogie boarding, kayaking, paddle boarding.
Swimming, but in designated areas in the summer. Watch for signs.
Fishing off of the Oceanside Pier.
Walking, jogging and bicycling on the boardwalk.
Summer outdoor concerts in festivals at Oceanside City Beach.

25 Best Things to Do in Oceanside, California
http://vacationidea.com/california/best-things-to-do-in-oceanside-ca.html

Oceanside Activities
http://www.oceansidechamber.com/things-to-do.html

Oceanside Restaurants
http://www.oceansidechamber.com/oceanside-restaurants.html

Swami’s Cafe http://swamiscafe.com/

The Flying Pig http://www.flyingpig.pub/oceanside-ca/

Ruby’s https://www.rubys.com/

Hello Betty’s http://hellobettyoceanside.com/

333 Pacific https://www.cohnrestaurants.com/333pacific

Davina's Cabo Grill http://davinascabogrill.com/

Tremont Bar and Grill http://www.tremontstsportsbar.com/

Carlsbad Premium Outlets
http://www.premiumoutlets.com/outlet/carlsbad

Oceanside has a premier, free to enter, off-leash Dog Park available to locals and guests alike. Located at 2905 San Luis Rey Rd, Oceanside, CA 92058 you will find a peaceful location for your dogs to freely run and play in the cool crisp beach city air.

https://www.bringfido.com/attraction/543

Oceanside has plenty of trail and hiking opportunities, and Fido is welcome to join you on a leash at several of those locations. The San Luis Rey River Trail is a great place to walk your furry friend, with a trail totaling 9 miles with mostly flat and smooth ground this is a local favorite. Guajome Park is another place that invites you to walk your leashed dog, with 4.5 miles of low traffic trails at your disposal.

https://www.traillink.com/trail/san-luis-rey-river-trail/

LEGOLAND California Resort has more than 60 rides, shows and attractions including the new LEGO Star Wars™: The Force Awakens Miniland Model Display opening March with a 16-foot long Finalizer model made completely out of LEGO bricks plus six more scenes! Conquer dragons, battle pirates and discover ancient treasures at the Resort, also home to LEGOLAND Water Park, SEA LIFE Aquarium; and NEW for Summer enjoy Surfers’ Cove – a water park expansion featuring racer-water slides. There’s so much to do, you’ll want to spend the night at LEGOLAND Hotel featuring new LEGO NINJAGO themed rooms.

https://www.legoland.com/california/

Oceanside, California offers triathletes a chance to race in the region that birthed the sport in the early 1970s in Mission Bay. Oceanside is located at the northern end of San Diego's North County, which includes the beach communities of Del Mar, Cardiff, Encinitas, and Carlsbad—home to surf, triathlon, and beach culture. Just half an hour south lies San Diego, with its world-famous zoo, parks, and other attractions to fill a spring break vacation. Just two hours away from the local mountains, skiing and snowboarding are other options.

http://www.ironman.com/triathlon/events/americas/ironman-70.3/oceanside.aspx