Seems an easy start to what can be a complex answer.
You need the things that would be important to you ... and that depends on the trip...
Is this a day trip? or would you be on holiday for some extended time?
Destination? Mode of travel? Who would you be traveling with, if anyone?
Are you on meds of any kind?
Depends if I’m traveling by car, boat or overseas! One thing I can say as an American I say the roaming fees will kill your wallet! So I now buy phones in foreign countries anymore with pay phones that don’t break the bank!