JetBlue, Alaska Airlines, and Delta have round-trip flights from New York (JFK) to San Diego (SAN) for $218, NONSTOP. Flights September to February. Weekends available. Origin/Destination reversible.
A Carry-on bag and also a personal item is allowed for free on Delta and Alaska Airlines. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.
JetBlue’s “Basic” fares only allow a small personal item (17 x 13 x 8 inches), like a Jansport SuperBreak backpack without a fee. A regular sized carry-on bag will incur a bag fee. To avoid bag fees, consider using the JetBlue Plus credit-card for the free checked-bag perk.