Passport is the only form of ID you need for this trip. Someone have mentioned above that the places don't stay open late???? In London you have access to everything around the clock, from 24 hours grocery stores to places to eat and drink. The best way to move around London is the "Tube" metro system that will take you any place around town. Map
HERE Watch your pockets on the tube and never carry a bag on your back. Plenty of pocket pickers around.
Prices compared to the US will look very high but just think as 1 to 1 and you will not get depressed :D $1.00 as £1.00. I hope you don't smoke because if you do it cost A LOT. However if you do, buy your cigarettes from here and take them with you. Marlboro is £6.20 for pack of 20, this is almost twice the price from here.
Money exchange...You can use your Debit/ Credit card in every place, check with your bank about charges, the best and most secured place to exchange currency is the banks branches, NEVER EVER change money at your hotel or in one of the hundreds street exchange offices. The rates are horrible. If you using debit card most US banks will NOT charge you any additional fees, so this is THE best way, not to mention that you don't have to carry coins around your pockets.
Use the provided safe box at your hotel room for your passport, return ticket etc. If you don't have one at your room you should use the one at the reception.
If you going to eat out ask at your hotel for local small restaurants, usually excellent food and the prices are 50% less than any fancy place.
Shopping...well if you going to end up in Harrods or Selfridges you need a good bank balance, I am sure you know that :D In Harrods you will discover THE best quality food bistro in the world but be ready to pay for it if you decide to have lunch or even just a snack.
This is all I can think off for the moment. Any questions just ask :D
I am sure you will enjoy your stay in London, have a safe trip and keep us posted ;)
