St. Vitus Cathedral
It's a really nice thing to see in Prague, definitely worth seeing! Only bad thing is that you have to pay entrance fees for every single museum or anything else that's of interest. Even in the cathedral they let you kinda see half of it, for the other half they charge you just to go there. The souvenir shops are nice, there's currently a kind of medieval theme in front of the castle that sells good looking and smelling food at reasonable prices. Would probably recommend taking a tour as there are some things that you won't even find if you just browse through the place on your own. There are many spots to take incredible pictures as well.