cheapest way of getting round London this weekend

[tescodisco]

I'm off to London this weekend with some mates, we're travelling down from Newcastle by train and will be getting into Kings Cross at about midday on Saturday.
On Saturday night we're staying in South Kensington, on Sunday we're staying in Hoxton (we managed to get cheap rooms in the Hoxton hotel promo).
On Monday we're planning to go to the Tate Modern and the London Eye, leaving Kings Cross at about 8pm to travel back to the toon.

What is the cheapest way of getting around? Normally I would buy a one day tube pass - but someone told me there might be a cheaper way to do it by buying a pass for a couple of days?

Any advice gratefully received!

[Equinut]

the oyster card is the london travel pass - last time i was in town (just before christmas) they were doing some kind of promotion on it, it might be worth googling it to find the london transport website and find out a bit more - not sure if it's better for longer-term stays though.

out of interest did you hit your target of getting that balance below £5k by the new year?

equinut