The Anchor is hard to get to - follow the narrow paths past Southwark Cathedral, past the ancient Clink prison. It's a great pub in the summer - it has a huge outdoor terrace overlooking the river. You can sip a drink and watch the boats, see St. Paul's and Tower Bridge, and chill out after a long day's activity (or idleness, activity isn't required as door policy).
From the terrace of this pub, the famous 17th century diarist Samuel Pepys watched the Great Fire of London race through the north bank in 1666. The pub itself is a 17th century building, as the south bank of the river suffered a serious fire of its own a decade later, and divided into a few, small bars - there's been a pub here for over 800 years. It's a very attractive building, with cosy seats, and low ceilings. The current claim to fame is that Tom Cruise was filmed having a quick pint here in Mission Impossible.
At night, the city is lit up and the vista is stunning - but bear in mind that even in the height of summer, it's a good idea to have a jumper or jacket for late in the evening, especially next to the river.
To get here, the nearest stations are London Bridge or Cannon Street (which is the other side of the river, you must cross the Thames to get here.)