Downtown San Antonio is walkable — the Alamo, the River Walk, Market Square, Military Plaza, and the King William District are all within comfortable walking distance of each other. The River Walk itself is the most pleasant way to move between riverside attractions. For the missions south of downtown, a car or rideshare is the practical option; the Mission Reach trail connects them for cyclists.
VIA Metropolitan Transit runs buses throughout the city. The VIA Streetcar runs a limited downtown loop on weekdays. Rideshare (Uber and Lyft) is widely available and inexpensive for most city distances. A car is useful for visiting Brackenridge Park, the McNay, and the Botanical Garden, though rideshare serves all three. Avoid driving on the River Walk level — it is pedestrian only.