A Super Off Peak Day Travelcard costs £36.90 and so this is the fare to beat.
A Super Off Peak Day Return to London Bridge (issued to London Thameslink) is £28.60
A Super Off Peak Day Return to London Terminals is £26.60
Contactless from London Bridge to Canada Water is £3.10 each way so £6.20 return
But it's the same cost with contactless from Kings Cross St Pancras, but you'd have to change to the underground there or exit St Pancras station with your paper ticket and re-enter using contactless, effectively stepping back a train).
An even cheaper way is to travel buy a Super Off Peak Day Return from Huntingdon to Finsbury Park, costing £25.90 and use contactless from there. You would need to exit the train from Huntingdon and touch in with contactless. The cost from Finsbury Park to Canada Water using contactless is £3.10 each way but if you avoid Zone 1 and travel via Highbury & Islington and Stratford, and touch the pink card readers when interchanging then the cost is £2.20 each way.
It really depends on how much you value time and convenience.