If you're taking long bus, train, plane or boat rides your phone battery is unlikely to survive the journey. Little portable batteries like the one on the right are perfect to ensure your phone doesn't run out of battery before you get to your location. Particuarly important if, like me, you use your phone to store screenshots of the address, and key part of the map for where you need to go at your destination. I particuarly love this battery pack as it has a little flash light in it too. Be sure to buy a few though as these little portable batteries will only last about 6 months to a year depending on how often you use them since they have a maximum amount of recharges.
The above battery packs are great becuase they can even fit in your pocket or a small handbag but if you're likely to be in places with unreliable power connections (it's not that uncommon to find yourelf in places with no plug sockets or frequent power outtages when travelling) taking one larger battery pack with you incase you need to charge devices over night is also really useful. I really like this one as it as two USB ports meaning I can charge more than one item at a time.
I was extremely unprepared on this front and only thought to buy a torch the day before I left for my travels so ended up having to buy a very small one from a sovenier shop in London. Needless to say it didn't last long but torches are essential. They will be your best friend in a powercut, if walking down uneven roads in the dark (such as at the start of treks) or when wanting to read in a dorm when all the lights are out. Be better prepared than I was and get yourself a decent one. I like the look of this set as it includes a head torch too which is even more useful for treks.
Most hostels will provide at least a small space to lock your valuables in, with many even providing a larger locker for your whole backpack as well. They frequently don’t provide padlocks, though, so be sure to take a couple with you on your travels. A lot of people recommend locking your backpack in transit. If you are going to do this ensure the packlocks you buy are TSA approved as this means customs can still open you back when flying.
Oh the inclusion of this one will make my parents so happy. The day I was leaving for my travels my mum rushed aroud saying "Are you sure you don't want to take xxx?". If I had packed everything she suggested I probably wouldn't be able to lift my bag let along walk anywhere with it on my back. One item she did suggest though, and that I rejected at first until my dad suggested it too, was a travel sewing kit. And they were right. Having a travel sewing kit is INCREDIBLY helpful. It doesn't take up a lot of space but means you can repair small holes in clothes rather than having to throw them away and the neddles also prove extremely handy when needing to remove coral/glass/sea urchins (yes all three have happened to me, multiple times) from your feet.