At the southern end of San Diego Bay are a series of salt marshes and mud flats that are favorite spots for local and migrating sea birds. On the Chula Vista side of the bay there is also a nature center that focuses on the environment, common marine species to the bay, and sea birds. On the Coronado side of the bay there are pull outs along the Silver Strand where you can pull over and watch the birds.
The South Bay Marshes are easy to get to. Take Interstate 5 south to the E Street exit in Chula Vista and make an immediate right into the Nature Center parking lot. After visiting the marshes on the eastern side of the bay, you can continue south on I-5 to California 75 and follow it around through Imperial Beach to the Silver Strand and then drive north toward Coronado. There are turnouts on the road where you can pull over and go exploring.
Travel safe. Have fun.