What honorarium is befitting for a guest speaker apart from money?
My answer to What honorarium is befitting for a guest speaker apart from money? on Quora
Here are a few ways you can treat your speakers really well.
Pay for their flights and a good hotel (with an extra night either side of the conference so they can relax a little).
Have a car (or an organiser in their own car) pick them up from the airport.
Lend them a smartphone with a pre-configured data SIM in it for the duration of their stay (even better: make sure wifi tethering works on it).
Throw a really good speakers dinner at an excellent local restaurant.