Mission Bay park is the largest man-made aquatic park in the United States. This park is 4,325 acres of water and land, over half of this area is water. On the weekends, you'll find locals here flying kites, picnicking, having family gatherings, sailing model yachts, canoeing, kayaking and bicycling. I come here twice a week for my morning boot camp classes.
If you don't have a kayak, you can rent one from here -http://www.missionbaysportcenter.com/ During the summer there are fantastic fireworks in the evening at Sea World, watching them in the water from a kayak is amazing!
This park is basically a huge area of water surrounding by grassy knolls with playgrounds and picnic tables.
My husband and I like to load our bicycles in the car to go bike around Fiesta Island, a large piece of land (actually dredged up from the ocean floor and was once an old garbage dump) located in the middle of Mission Bay park. There are many running & bicycle races held here, but the most popular is the 'Over the Line' tournament. This is a rowdy softball event held for charity, you can read more here - http://sandiego.about.com/od/outdoorrecreation/p/otl.htm
This is a great park to visit for enjoying the wonderful San Diego weather and sunshine.
Tip: in the summer, this park fills up really fast, so if you want to visit make sure you get here before noon or you'll have a hard time finding parking.