I was traveling from the US. Here are a few things I learned that I don't see already mentioned above:
-Wireless is everywhere and advertised as "free," but typically requires that you have a cell account with either "Orange" or "O2," the two major cell players there. After struggling to find truly free wifi I went ahead and paid for a one month subscription with Boingo. I was actually using my iphone (sim password protected to prevent cell usage) so the cost was $7.99, but for a laptop the charge is $9.99 for one month. Even if you won't be there for a month this is a relatively small amount to pay for wireless access all over London. There is no contract and I canceled the account as soon as I got home.
-If you want to call back to the US I suggest Skype. For under $3 you can make unlimited phone calls back to the US for a month. Again, you can cancel this subscription once you return home.
-Get Tube and Bus Maps! Despite popular opinion the tube system is clean and safe and the double-decker buses are fun to tour around in. You can go to a bus or tube ticket counter and get day passes for $20US (You pay in British Pounds, but I'm converting) that let you use the mass transit system in the downtown London area all day. Check with the ticket agent to find out if there are cheaper or extended offers for multi-day passes.
- Be wary of paying for a "London Bus Tour" I took one of these for 20BP and didn't realize the buses stop running at 4:30 PM. I ended up getting stranded outside of Buckingham Palace and had to pay tube fare back to where I started. These tours aren't a bad idea if you don't know where everything is or don't want to have to worry about navigating the public transit system to see all of the sights.
As for things to do, well, I think I either did or saw everything worth seeing. If you are a history buff you can spend literally months reading all of the historical markers throughout the city. For reference my list of must-see locations can be found here (via a facebook photo album) where I took pictures of my daughter's toy giraffe everywhere I went: http://www.facebook.com/home.php?#/album.php?aid=30617&id=1629570049