St. Paul's Cathedral has something for everyone! For those who are of faith and would like a taste of worship in an Anglican church, the cathedral holds various worship services daily (usually holy communion, along with other services, depending on the day). I never personally got to go to a worship service, but I have heard from others that the services are really beautiful.
For those who are not interested in the religious aspects of St Paul's Cathedral or don't have time to go to a service, the architecture and historical value are both definitely worth stopping by for. The cathedral has a history dating back to 640 AD, and the current building of the cathedral has been there since the late 17th to early 18th century. Each day, the cathedral is open from 8:30am to 4pm, and for a small fee (£2-3), one can elect to take either a guided tour or an audio tour of the cathedral. I took the guided tour, and it was very informative.
One exciting thing about St. Paul's that I must mention is that you can take stairs and walk around the base of the dome in the interior of the cathedral. If you really feel like climbing a lot of stairs, you can also go to the top of the steeple, to a little walkway outside, and get an awesome view of the city. Getting to see the city from the top of the church is probably one of my fondest memories from St. Paul's.
While St. Paul’s can be called a "tourist trap" if you go during the times that tours are offered, it really was not that crowded when I went. It was a beautiful sight and a great historical lesson of London.