I'd be tempted to go for the 1 day Travelcard.
(Tube, DLR, Overground, buses, and various National Rail services)
Zones 1-4 should sort you out for the majority of places you want to see or travel to.
If u go "off peak" (after 9.30am in the morning) it's only £7.70
http://www.tfl.gov.uk/tickets/14416.aspx
