The season doesn’t start in Canada, it starts in Bahrain. Usually Bahrain or Australia are the openings rounds lately, though it has been different over the decades.
Depending how strict most drivers’ contracts are, some are allowed to race or do other things. Though most will spend a little time off with their family or friends, then it’s back to probably daily visits to the factory for meetings, design updates, progress checks, simulator work, PR commitments, and physical training.
I would imagine most teams are doing a lot of running on the simulator to prep for the new aero regulations and make sure they are getting correlation between their CFD/wind tunnel/and how the car is performing in the simulator. There is going to be a lot of new things to learn on setup and driving with the new aero regs.