Trento isn't very big and can be 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, a large square where the cathedral, the bell tower, and Neptune's fountain are located.
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 across the river to the village of Sardagna. From there, Trento, the entire valley, and the mountains surrounding it can be seen. The panoramic views are magnificent.