The Cloth Hall ("Sukiennice") is the historical building located in the Main Market Square, in its very centre. Here existed some depots in the past, mostly with cloth. Cloth Hall prototype in the form of stalls already existed in the 13th century.
The building (A; GPS: 50.061703, 19.937394) is beautiful and it's lovely and cool inside if you are looking to escape from the sun a little. Really nice stalls inside with a variety of tourist items for sale, so if you're looking to buy a Polish souvenir then this is the place. You can choose from a huge variety of goods including lovely hand made jewellery, leather goods, pottery, glass and a plethora of interesting goodies. Search for a good deal, some of the stalls can be a bit pricey, but all lovely to look around.
Next to the Cloth Hall there is a statue of Adam Mickiewicz (B; GPS: 50.061480, 19.937984), set here in 1898. On the same side of the building in 2010 was installed a fountain (C; GPS: 50.061919, 19.938283) with a glass pyramid in the center of it. The fountain is a controversial thing for many people, unmatched to other monuments located in the Main Market Square. And under the Cloth Hall we can visit a very interesting underground museum: The Rynek Underground. On the other side of the Cloth Hall there is the town hall tower - a Gothic building from the 14th century (D; GPS: 50.061500, 19.936476).
Also be sure to check out the Cloth Hall (and the Main Market Square of course) during the day and at night. It is a hustle and bustle place during the day. At night it is a bit more peaceful and the lighting on the building and the entire square is just magical.