St Pauls is a spectacular tourist destination and sight even from the ground and when you can peer over the top of the cathedral dome as the sun sets behind it, you know you're in for something special.
Take the lift up from the ground floor to madison bar (level 6). The glass elevator on the way up is pretty cool in itself as it goes quickly almost like it should be in a theme park.
Once you step out of the lift you'll immediately see the viewing area. The St Pauls viewing area is huge so there's plenty of room for you to take some selfies with the beautiful St Pauls Cathedral and a setting sun in the background.
From the madison bar roof you can see all around London too, from the London eye on the shore of the Thames, all round this is a great free viewing spot to soak up the beauty of London city.