For this kind of trip, it is useful to have two bags. A Backpack (Surprised, eh?) and a Daypack.
Choosing the right Backpack (if you don’t already have one) is an important part of planning. See my other instructable for more details on shopping for a Backpack.
Here is what I find useful in a Backpack:
30L to 36L
2 access points to the main pocket
mesh back system for maximum ventilation
For your daypack, the best is to choose a bag that is versatile and easily storable. It doesn’t have to be a rucksack; it can be a messenger bag or a purse. This is the bag you will use when you want to go touring without carrying your big backpack or on a day hiking trip. Choose something that can be stored in or on your backpack when not in use. Carrying two bags gets annoying pretty fast.
Practicality is very important, but pick something you also like. I’ll come back to this a little later.
*If you do use a rucksack, I’m begging you: don’t wear it in front of you. It’s like wearing a fluorescent pink and fluffy hat with glittery letters screaming I’M A TOURIST. The best way to avoid being robbed when in touristy areas is to blend in. Now this doesn’t apply if you’re in a big crowd or transport system, just if you’re generally walking around the streets.*