Chiltern Railways now have new benefits for Travelcard holders who buy
a Season ticket from them.

"If you buy a monthly (or longer) season ticket to London or a monthly
London Travelcard season ticket from any Chiltern station, you now get
FREE weekend travel on all Chiltern Railways services across our
network (just show your valid season ticket onboard to qualify). If
you buy an annual season ticket, we'll also send you 6 FREE off-peak
day passes for you or your friends/family to use on Chiltern
services."

This seems to be the best set of extras in the London Travelcard
extra...

B.