Use an Oyster card. It charges you the single fares for your journeys until you reach the amount you'd pay for a daily travel card, then stops. If you make only a couple of journeys, it's cheaper than the one-day card.
Whatever you do, don't buy it on line or with a credit card. All your journeys are monitored! Buy one for cash at the first station you go to, and pay cash at any machine in any station to top it up. If you get it right, you'll be left with nothing on it at the end of the day. Also, it's the fastest way through the stations as all now have auto barriers for Oyster cards. Put it in your wallet close to the edge, and all you need do is pass your wallet across the reader - no need to get it out of the wallet. If you are carrying a bag, put it in the bottom and pass the bag over the reader - easy peasy!
Have fun in the big smoke!
