Assuming that you arrive in London after 0930 on Friday, all of your tube travel will be at off-peak rates. One-day travelcards cost �4.90 per day for unlimited travel within central London:
http://www.tfl.gov.uk/tfl/fares-tickets/2006/o ne-day/travelcard-day.shtml
Alternatively you could buy an Oystercard. That way, you'll only have to queue once at the ticket counter. Off-peak travel is capped at �4.40 per day for off-peak travel within zones 1 & 2. Additionally, there's a �3 initial fee for the card. (That's actually refundable but it's simpler to just hang on to the card for future use).
So, you can either queue up 4 times and pay a total of �19.60 for one-day travelcards or queue up once and pay �20.60 (including the refundable �3 fee) for an Oystercard.
Personally, I'd opt for the Oystercard.
Chris