Tucson is built for cars. Distances are real, parking is plentiful, and many of the headline sights are 15–40 minutes apart by road. A rental from the airport is the practical choice for most visitors.
Downtown, the University, and Fourth Avenue are connected by the Sun Link streetcar, which runs every 10–15 minutes — free zone within downtown. The streetcar is the easiest way to move between Hotel Congress and the university museums without finding parking.
The Sun Tran bus network covers the rest of the city but is slow. Cycle infrastructure has improved a lot in the last decade; The Loop is a paved 130-mile river-trail system that lets you bike across most of the metro on its own dedicated path. In the heat of summer most locals walk only first thing in the morning or after dark.