>>2798392
Get something with a 40L capacity with a "clamshell" type opening (pic related; one side opens all the way as one giant flap).

Other features to look out for:
>sternum & waist straps if you plan on walking for extended periods while wearing the backpack
>lockable/"kissable" zippers (two zippers with a hole that can be locked together with a padlock)

Optional: Get a second, smaller bag (10L or less) to use as your "daypack."
Your main 40L backpack carries everything from the airport to your ho(s)tel, while your daypack just carries your daily essentials (camera, water bottle, power bank, snacks, pen, medication, etc.)

Not gonna mention any brands because I'm not a shill, and I don't want to restrict your inspiration too much.