Trento isn't very big and can seen fairly quickly. Strolling around the city center is very pleasant because the architecture is historic and very appealing.
One of the highlights is the Piazza Duomo with the cathedral, the bell tower and Neptune's fountain.
Another point of interest is the large Castello del Buonconsiglio which houses the Provincial Gallery of art.
The most interesting thing to do in Trento is to take the cable car to the top of the mountain to the village of Sardagna. From there Trento, the entire valley, and the mountains surrounding it can be seen. The panoramic views are magnificent.