Tecate is a small town 30 minutes east of Tijuana. It is also easily accessible from San Diego by driving east, then crossing the border. It is much more low key and slower paced than Tijuana, and offers more of a "Mexican" experience for visitors.
There isn't a whole lot in the way of entertainment, but there is a mercado and a park where you can relax and people watch, or just browse around, or even visit some of the eateries. Sometimes on weekends there are performers in the plaza.
If you just plan on walking around the main center, you can park on the US side and walk across the border, however if you want to get around a little easier you can just drive across. Remember to get Mexican insurance though!
Tecate is also home to the Tecate Brewery, which offers tours and free beer.