I keep it a habit to have my backpack or a messenger’s bag with my laptop and important things and a smaller bag with food/snacks/water(the small lululemon reusable bags are great for this) if I’m riding the train or flying Southwest(or if I’m checking my carry-on for bigger liquids).One good bag to have is a personal bag that is small enough to keep by your side whether you are flying or on the road. That bag can carry passports, medicine, and whatever essentials you might need. On a plane that saves you from having to get up and retrieve your stuff from the overhead bin midflight. Also convenient on a train or in a car.
My Costco suitcase is great for checking in or carrying on. It’s a little bigger than the smaller roller bags I’ve seen people use on Spirit(who’ll even charge you for using the overhead bins).