Paul Corfield wrote:

Or simply purchase a Mobilis ticket which is their version of a One Day
Travelcard. You get a form of "ID" card which you keep and then simply
buy a magnetic ticket as necessary. You then date it and write on the ID
card number.

I used one of those a few years ago, but not on buses. It was good value
for the trips I made on the day, and I was surprised that it isn't
publicised to tourists (or not noticeably). They plug the Paris Visite
pass in the tourist literature, but not Mobilis.

--
John Ray