Central Tijuana is navigable on foot — Avenida Revolución, the Centro, and much of Zona Río are all within walking distance of each other. For longer distances, taxis are plentiful and inexpensive; agree on a price before getting in, or use Uber (which operates freely in Tijuana). The local buses (called rutas) are cheap but complex for newcomers.
Driving across from the US involves delays at the border on the way back. Many visitors park on the US side at San Ysidro and walk across, then use local transport or taxis in the city. This is often the most practical approach for day trips.