When my canadian cousins came over we took them on every public transport possible. You just need a 'contactless' credit card for every traveler and it will automatically stop charging you once you have reached the daily max. No need to bother with Oyster cards.
Bus
River Bus
Tube
Overground
Cable Car
DLR
The easiest way to get around Central London is tube & maybe Uber in the evenings.
Have fun
