Catering to American tourists for years, Tijuana is a party place and the fourth largest city in Mexico. It also has a surprising amount of history attached to it and lots of fun sights to visit. You'll probably get swindled at least once by someone who insists on calling you "friend", but it'll be worth it for the stories and the fun you'll be having. Indulge yourself and spend some time wandering around the historic Avenida Revolucion, visit the Playas, and check out a horse race.
Avenida Revolucion: This is the main tourist street, filled with bars and resturants and curio shops.
Centro Cultural: Visit the Museo de las Identidades Mexicanas and the spherical Omnimax Cinema.
Catedral Nuestra Senora de Guadalupe: The main cathedral at Calle 2.
Jai-Alai Palace: Games are no longer played here but it's still worth a visit, especially if you're in the market for souvenirs.
Plaza de Toros Monumental or El Toreo: Charreadas, or bullfights, are held at two rings, one on the beach-front and the other a few miles east of downtown on Blvd. Agua Caliente.
Agua Caliente race track: Horse (Saturday and Sunday) and greyhound (Monday through Wednesday) racing.
From San Diego: Just walk across! The Blue Line Trolley in San Diego takes you right to the border -- just ride to the last stop in San Ysidro, hop off, and follow the signs to Mexico. Keep in mind that this is the one of the busiest border crossings in the world, so if you're driving, give yourself plenty of time and try to avoid peak hours (e.g. rush hours and weekends). The border is open 24 hours a day.
Bus Station: Many long distance buses leave from Calle 2 near Revolucion. The Central Camionera is out near the airport at Lázaro Cárdenas 15751. You can take a taxi or local bus (school bus) or collectivo van there.
Summer weekends - Charreadas (rodeos) at Blancarte and Calle 2.
Weekends - Street Market at Avenida Riveroll and Calle 7.
January to March - Whale Watching: Gray whales pass the Ensenada Bay on their way north from their calving grounds further south. Boat trips can be arranged.
February (Lasts 5 days prior to Ash Wed.) - Carnaval with parades, dances, rides and games, as well as food and drink stalls.
April 30 - Rosarito - Ensenada Bicycle Ride
April - Annual Newport - Ensenada Yacht Race
June - Baja 500 Off-Road Race
Last weekend in June - Estero Beach Volleyball Tournament
August - Annual Fiesta de la Vendimia: 10-day wine harvest festival.
Sept. 24 - Rosarito - Ensenada Bicycle Ride
October - Annual Fiesta del Tequila
Baja 1000 Off-Road Race