If you find yourself in Trento without a car, don't worry, there are still plenty of options available. The train station seems to be the transportation hub for the city, and buses and taxis are coming and going throughout the day.
Buses are an economical way to get around, and to get to know the city, some of its frazioni and even some of its residents. In general, most of the 18 or so bus lines make their way through the city, on their way to or from one of its suburbs. Every bus line has a stop at or near the station at some point on its route, so from the station, it's easy to find a bus that's going your way.
Link to: Trentino Trasporti
Tip: When riding the bus, be sure to validate your tickets on board. The 'bus police', as I call them, will board every so often to check your credentials. If you're not legal, expect a fine.
Tip: Students use the local buses to get to school, so when summer vacations start, schedules will be scaled back.