San Francisco's most visited attraction is a two-level waterfront shopping and dining complex built on a former cargo pier. It is unabashedly touristy — souvenir shops, chain restaurants, and street performers — but it earns its place on the itinerary for one reason: the sea lions.
In 1989, shortly after the Loma Prieta earthquake, California sea lions began hauling out on the docks at the pier's western end. Nobody planned it. They just showed up, and now a colony of several hundred lounges, barks, and jostles on K-Dock year-round. Peak numbers are around January; summer sees fewer. The colony is free to watch from the upper deck.
The views from the pier are genuinely excellent — Alcatraz directly ahead, the Bay Bridge to the right, and Marin headlands on clear days. For food, skip the sit-down restaurants inside and head to the crab stands outside along the Embarcadero instead.