We've had great luck in Europe with a couple of different types of electrical plugs with this one that I bought through Amazon:
World Travel Power Plug Adapter with Dual USB Charger - M8 By OREI - The International Mate
by Orei. It would be worth checking on the countries you plan to visit for their electrical plug style. This one has many.
I have an EBag TLS MotherLode Weekender, Convertable (I used it as a backpack comfortably, but we did not do any hiking).
My husband used his Tom Bihn Aeronaut 45. It can be used as a backpack, but he prefers to carry it over the shoulder. I agree with previous posters....pick a backpack meant for hiking if you will do any decent amount of hiking. The comfort needs are different for hiking than for train/bus/plane/walking to get around. A reminder: if you are planning any overseas flights, especially on "low cost" carriers, their size and width limits for carry on bags are often much, much lower than what the "big" airlines allow. Even on US flights, keep in mind the smaller overhead bin size on regional jets. When you choose flights online, click through to look at the type of plane (and seat options: a site like SeatGuru.com can help). Pick your best option. It is wonderful to have your bag with you for flexibility for changes, and it is nice to be able to have it with you in the cabin (rather than gate checked), if you have a tight connection. I love the packing cubes someone mentioned, but do realize that they add a little weight if you will be hiking for long.
I second the suggestion of looking at PacSafe.com. I carry a CitySafe crossbody bag as my "personal item" and carry electronics, etc. in secure sections. As well as tissues, glasses, bottled water etc. in easy access pockets. Check PacSafe for backpack net locks.
Also...FOR SURE get a good money belt to wear around your waist, under your clothing. They even come RFID protected. We keep all of our travel documents, our passports, each other's passport copies, money, credit cards. etc. Consider wearing it while you sleep and keep it near you (in a zip lock bag) when you shower. While this may seem crazy paranoid re: theft...I actually do it because, over the years I have had to evacuate my hotel room several times for different reasons (real fire, false fire alarm, power outage). If you can take nothing else, you want to have your documents, money, credit cards with you in an emergency. For the same reason, I put my money belt on right after I get past security at an airport.
Take reasonable precautions, you'll have more fun.