Strahov Monastery
Super cool library. It's definitely a quick trip. If you want a closer more in-depth experience you will need to book a tour. There is a link to booking big tours on their website. If you don't have a lot of time it's still worth popping in. It's about $5 to get in. Another $2 if you want to be allowed to use your camera (in non-flash mode). It's a pleasant walk / loop to go to the monastery, swing by the Loreta Church and then continue down to Prague Castle. You can also do the walk in reverse.