London - 2 days in London

Day time highlights:

Tripadvisor is a very good tool for trip planning, especially for HOTEL selections and tips for getting around the main attractions.

Transportations & Sightseeing:
   You could take one of those Hop-in-Hop-off Tour Bus (Day Pass)                 
           - very convenient, most of them have 8 stops at the most popular tourist attraction                     
           - but you could get stuck in traffic at rush hours

   You could take the local bus #9 - self-guided sightseeing tour bus  (cheaper)
           - you will pass by many tourist attractions as Harrods, Big Ben, St. Paul Cathedral, Tower of London and Kensington Palace 

                       

Metro system - London's metro system is very convenient
                                   - Their metro staffs are very nice and helpful (unlike Paris)

Walk - If you really want to feel the city, scrolling around London is not a bad idea, the top attractions were pretty far apart though

Cruise - Take a River Thames cruise/city cruise from London Eye to the Tower of London
             - Or take a River cruise from the Tate Modern to Tate Britain 
                                                   (Tate to Tate Boat ~£5.50 single, £11 return)

Taxi - We had bad experience taking taxi back to hotel from the downtown area.  We showed the taxi driver our hotel address, we showed him the map and the GPS directions from Iphone.  He still took us to the wrong area, and we ended up walking 30 minutes back to hotel.  Unless you book the taxi ahead of time, avoid taking taxi as a tourist.

 

I only had two days in London, here was the original plan for the two days:

Day 1:
 - Westminster Abbey (opening hours: 9:30am - 2:30pm (for sat))
 - walk through St. Jame's Park
 - Buckingham Palace 
            - watch Changing the Guard (make sure you check the schedule ahead of time)
 - House of Parliament, Big Ben
 - Tate Gallery
 - Thames River Cruise to Tower of London
 - Tower of London
 - Tower Bridge

(optional): St. Paul Catherdral + Millennium Bridge
                 Trafalgar Square + Covent Garden (Department Mall at the West End)

 

Day 2:
 - breakfast at Hotel
 - Victoria + Albert Museum (free admission)
 - Harrods (Knightbridge) - Afternoon tea at Georgian Restaurant - Book ahead
 - Shopping around at Mayfair
 - London Eye (enjoy the sunset view or the night view)

Our hotel's sunset view from the Lounge :)

Our actual 2 days:
  We were tired from the flight on our first day, we had a very chill first day.  Touring around the main attractions via a private car, shopped around at Regent Street, lunch, back to hotel and spa.  
  Second day was a power walking tour: walking to all the main attractions: Big Ben, Westminster Abbeys, Buckingham Palace, St. Jame's Park, south bank, London eye, Olympics sites, Millennium Bridge, Tower of London, Mayfair...etc etc......It tells you that all the main attractions could be visited in one day (a little congested but it was manageable :)