Holocaust Museum at Pinkas Synagogue
The visit here is very sobering. Inside the synagogue, which has been turned into a museum for the 77,297 Jews from Czechoslovakia killed during the Holocaust of WWII. If the names aren't sobering enough, the museum also houses a permanent exhibition featuring the drawings of children who were locked away in the Terezín concentration camp.
The Pinkas Synagogue is the second oldest surviving synagogue in Prague, dating from around 1535. The synagogue is subject to flooding and has had to be restored multiple times over the years since then.