If you're arriving at LHR on a Friday and flying back from LHR on a Sunday, then you may be best off buying a three day Travelcard for zones 1 - 6. A three day card for off peak travel (you must start your journey on Friday after 0930, but can travel at any time on Saturday and Sunday) will cost £21.20.
A one day off peak Travelcard for zones 1 - 6 (again, presuming you're flying to LHR) costs £7.50. A one day zones 1 and 2 Travelcard for Saturday costs £5.60; again you'd pay £7.50 if you wanted to do a bit of sightseeing on Sunday before leaving London, buying a zones 1-6 card. Total cost £20.60, so not a huge difference there.
You can buy three day Travelcards at any of the Underground stations at LHR; cards are good for unlimited use on the Underground, DLR, London Overground, National Rail and bus services.