There's the one that SD Pokemom posted, which is about 30 minutes away from the hotel. The majority of the league is non-competitive juniors, though that might change when more people come out for Worlds. There are a few competitive masters.
There's a league in Temecula, California at a store called HobbyTown USA (not sure about the address, but if you look up hobbytown usa on google maps you should be able to find it). I've never actually been to this league, and it's around an hour away from the hotel, but IDK if you're staying with someone that lives close to it it should be worth a try.
There's one that started after Worlds 2009 in La Mesa, California. It's only about a twenty minute drive from the hotel. 5500 Grossmont Center Dr. La Mesa, CA, 91942. It's in the mall's food court. I've never actually been to this one, but for more info you can PM the league leader (mamaschutte) though i'm not sure if she comes on here much anymore.
My personal favorite is the one at Game Empire. (7051 Clairemont Mesa Blvd #306, San Diego, CA). It's only 15 minutes from the hotel. Every sunday starting at 11. It's run like an actual tournament. After swiss rounds the winner of each division gets a booster pack and everyone who participates gets a promo card. This is one of the smaller leagues in San Diego, but there are several good players who attend regularly. The tournament structure is a great way to prepare for Worlds. I recommend coming to this one, it's a lot more fun than the regular kind of league.