I’m taking my holidays just before or after my conferences, and using most of them to travel there, mainly by train.
The travel is also used to do my research work in math.

Many difficulties, especially to get refunded when you have so many different tickets, and such weird dates.
Since travel sometimes include holidays, I don’t really care about being refunded each time.

Some problems at ground borders, where Customs is less used to some kinds of visas/authorizations than in airports.

Also, I refused some invitations to participate in conferences, when they were far and I could not stay there longer. I’m starting to be understood by some colleagues.