I will only be in London for two days and plan to take the Tube no more than 2 or 3 times, so it doesn't make much sense to buy an Oyster card.
You can buy single journey paper tickets for the tube, but they strongly discourage it through pricing. I just looked up a zone 1 Tube fare and it was £2.40 on Oyster but £4.90 on a paper ticket.
At those prices even if you don't get your card deposit back (or you get the visitor card with its non-refundable charge) it doesn't take very many journeys for Oyster to beat paper tickets.
Another option is to use a contactless credit/debit card but if you have a non-UK card beware of foreign transaction fees.
Also note that while you can use paper single tickets on the Tube you can't use them on buses in London. Your only options there are Oyster, contactless and travelcards.
(I read there is an activation cost and you won't get a full refund if you use it for such a short time).
There are two types of Oyster card. Regular cards and Visitor cards. Regular cards have a £5 refundable deposit, Visitor cards have a £3 non-refundable "activation fee". The Visitor card can only be obtained outside London. Regular cards can be obtained at any Tube station. The Visitor card comes with a few special offers for London attractions that the regular card doesn't but otherwise the cards are much the same.
AIUI with cash at a machine you can top up any amount down to the smallest coins the machine will take while if you pay by card at a machine or pay at a ticket office the minimum top-up is £5.
Getting the deposit and remaining credit refunded used to require dealing with a manned ticket office but it seems it's now possible to do it at a ticket machine.