A must-visit in Prague is definitely the Klementinum because it is home to two of the prettiest Prague sights; the Astronomical Tower and the Baroque Library! You have to take a tour to be able to visit both. I advise you to buy the tickets online in advance because they tend to get sold out on the day!

The best way is to do so here: https://gyg.me/2Ji6S4qG (affiliate link)

During the tour, you get to see the stunning Baroque Library Hall, which houses an impressive collection of centuries-old books. You may view the Baroque Library but not enter it as it is extremely protected and preserved. You also get to visit to the gorgeous Astronomical Tower, where you can enjoy panoramic views of Prague's Old Town. It's a fascinating exploration of architecture, history, and knowledge, right at the heart of Prague. Fun fact; Mozart actually visited and played at the Klementinum!