If you have guest coming to visit and they want to know which Oyster cards to get, here is what I have learnt.
There are no paper travel cards any more, but you can one day, 7 day or monthly travel cards. The one day travel card is the same price as the price cap on a pay as you go oyster, and so is useless. The 7 day travel card (for zones 1-3) is definately worth it for tourists. If you do more than 11 journeys in a 7 day period then it's paid off. Even if you only travel for 4 of those 7 days, but do 8 Zone 1-3 journeys, and 5 Zone 1 journeys, then it pays off.