Some of the biggest attractions in Potosi are the mines. Potosi was founded because of the discovery of silver there; today the mines are still functional and there is an opportunity to visit the mine run on a co-operative. It is a shocking and memorable experience.
The churches and cathedral are well worth a visit; the architecture is amazing.
The Casa Real de la Moneda, the royal mint, is also a popular attraction; it has one of the best museums in Bolivia, and it even has the county's first locomotive.
The areas around Potosi are beautiful and worth the hike to get around them in the high altitude.
Quick Tips:
Be prepared for the altitude. Although we had already been in La Paz, the change still shocked us. Especially as the city is all hills, you notice them!
Best Way To Get Around:
If you have lots of luggage with you then a taxi is worthwhile. The buses are crowded, and so difficult to get on, and walking will take it out of you.