Get in earlier
If you're not too tired, then go into work an hour earlier to get a headstart on catching up on what you've missed. You'll probably want some quiet time before fellow coworkers come in and start asking you about your holiday or requesting you to do certain things, so try to get in before they do so you won't feel overwhelmed.
Write a to-do list
One of the first things you should do when you come back from a trip is to draft a to-do list for the day. You will probably have a million tasks to do, so note them down and figure out which ones you should prioritize. You'll feel better once you start crossing each one off.
Deal with your emails
It's time to tackle the mountain of emails that have been piling up in your absence! Don't try to respond to them all at once, because it might be overwhelming. Delete the ones you don't need, mark the ones you need to respond to later in the day, and reply to urgent emails that need an immediate response.
While you were absent, the office probably continued to chug along without you. Check on how the projects you're working on are doing and if there are any new developments. If someone was covering for you in your absence, then review what they've done. If necessary, make a note to give them feedback.
Check in with your boss and coworkers
Check in with your boss and coworkers as a courtesy to let them know you're back. Ask them if there are any new developments that you should be aware of.
Take it slow
The first day should be spent doing regular job duties as well as catching up on what you've missed. Try to refrain from taking on new tasks and projects on your first day back so that you're not putting too much on your plate.