Even light packers sometimes need a second bag. You don't have to "one bag" to travel carry on only.
Most airlines' basic fares allow for one carry on and one personal item. Your carry on bag goes in the overhead compartment. Your personal item goes under the seat in front of you.
What's considered a personal item?
The most important thing to remember is that your personal item must fit under the seat in front of you.
Your personal item must also be a bag, not a suitcase. Remember that your personal item is your second bag, not your main luggage.
Most airlines allow small backpacks, purses, laptop bags, tote bags, and briefcases to be carried on as personal items.
Airlines' rules vary on the size and weight of your personal item. Some airlines don't publish measurements at all. Some offer vague guidance. The rest vary in what size of personal item is allowed.
In the US, Spirit and Frontier—both budget airlines—allow personal items up to 18 x 14 x 8". So does American. But United and Jetblue both have smaller size limits.
The most important rule is that your bag must fit in the underseat storage, even if you don't know how big that seat is. Always check with your specific airline before flying.
Size and weight aside, look for a bag that will work well as a personal item. Even frequent travelers who invest in their main luggage often overlook their personal item. Bringing whatever small bag you have around the house might be okay for a short trip, but that bag won't make for an ideal personal item for travel.
A good personal item should be small, organized, and easy to access.
Compact size is important so that you can always trust that your bag will fit under the seat in front of you and not attract the unwanted attention of a gate agent or flight attendant.
Your personal item must have multiple pockets and some basic organization. You may neatly pack your carry on, but we all use our personal items as a "catchall" bag for everything else. Multiple pockets allow you to organize your small stuff, keep your electronics separate and protected, and carry a water bottle.
Lastly, your bag's pockets and layout should give you easy access to what you need during your flight. Stow your carry on in the overhead bin and use your personal item for everything you need during the flight. You'll already have it at your feet. Use your carry on to avoid having to get up, open the overhead compartment, and open your carry on.
A personal item is perfect for inflight snacks, headphones, an ereader, a light jacket, or any medications you might need. If you're seated in the middle or window seat, you can get anything you need without having to disturb the other passengers in your row.
The final consideration for a personal item is whether you plan to fly on a basic economy fare. Basic economy is a new fare class with lower prices but more restrictions including on your luggage. Most basic economy fares will only allow you to bring a personal item, no carry ons. Your only bag must fit underneath the seat ahead of you.
For basic economy fares, airlines do publish specific size limits. Make sure to follow your airline's rules as they are usually more strict with passengers on these budget fares.
If you're flying basic economy, you will want as large of a bag as you're allowed since that's all you will be carrying. A backpack or duffle bag will be best for carrying your clothing. A small laptop backpack will have the best balance of packing space, organization, and protection for your computer.
The Outbreaker Laptop Backpack is the ideal second bag if you're carrying a suitcase or duffel as your carry on.
This personal item backpack has plenty of pockets and organization so that you have a place for everything. The padded laptop and tablet sleeves will protect your gadgets. The laptop bag is also weatherproof thanks to the VX21 sailcloth fabric and coated, YKK zippers.
The Outbreaker Laptop Backpack is a larger personal item so it's also a good choice as your one bag when flying basic economy.
The Outbreaker Daypack is a lightweight, packable personal item. Carry it as your second bag when you need more packing space. When you want to travel lighter, the daypack packs flat into your carry on for easy, one bag travel.