Is travelling to London something that you have considered but not yet tried?
If you do visit London then you should definitely look at including some day trips on the River Thames as to not do so would be a great shame since you would be missing out on one of the iconic parts of London from a unique position, the river.
Travelling by river can easily be considered to be one of the most relaxing and comfortable ways to see the sights of London from the Thames. Travelling through London in this way is a unique experience and will let you take in all the sights such as the House of Parliament, Big Ben, the London Eye to name but a few places. River travel actually makes the experience different for you too, so even if you have already seen many of these places, seeing them from the river Thames from the deck of one of the many tour boats that populate the London River is an experience to be recommended.
Many of the cruises along the Thames can be pre-booked; you can also show up on the day and pay as you get on board. This is entirely up to you of course, but since the river trips may take in different sights depending on where about they depart from as well as which direction they are travelling, doing some research will mean you get to see what you want to on you river tour.
As an idea, you can make the river cruise day trip part of you sightseeing for day, rather than travel to the London destinations by tube, bus or taxi, you can incorporate the points of interest in the river tour, so when you get off the boat you can continue your sightseeing from a different part of the Thames.
Bear in mind the times of year that you will be visiting London, sometimes out of season, for example Jan-April and Oct-December the weather can be quite cold, to this end you should plan to take a cruise that has plenty of shelter on board, most cruises do have this but some may well be completely open-top which unless you wrap up warm might make the river tour less enjoyable.
Another interesting possibly is to hire your own river craft to travel on the Thames. Private hire boats though can be quite expensive, ranging between £900 and £1500 which is quite a lot, but if you are organising a large party if people it may well work out more cost effective.
Seeing London by night from the Thames is also something worth considering, many boat cruises will be available in the evenings; some have buffets or full silver service so you can eat while you sightsee. Live entertainment and music is also available on some cruises, so no matter what you want, there is something for everyone.