The cathedral is the burial place of all Russian Tsars from Peter the 1st to Alexander 3rd, with a couple of exceptions. Even the Queen Dagmar ( also known as Maria Feodorovna buried in Denmark, Roskilde Cathedral ) was brought to the Peter and Paul Fortress to be reinterred next to her husband Alexander the 3rd.
From the fortress walls you have a view over the sandy beaches that have become very popular in St. Petersburg that in summer become very overcrowded.