Oh sorry, I was confused. Although I bought an Visitor Oyster-card when I was in London 1,5 year ago, if you load it with like 20 pounds, you can have unlimited travel for £8.40 a day, as it doesn't charge you more than that. And if you need just a single fare the next day, you do not have to buy a paper ticket or another travelcard, which is cheaper in the end. Saying that you would mostly stay in Zone 1 and 2, if you somehow might end up in Zone 3 or 4, it only adds up to £10.60.
But buying a Day Off-Peak Travelcard would work too, but I prefer the Oyster-card myself :-)