Rain-wear for you and your backpack
In case it gets rainy or you have to put your backpack somewhere that you don’t consider “dry enough”.
Additional money in a hard currency
I always have some USD and EURos hidden somewhere in my backpack and/or clothes, which I don’t use for “daily payment”. This is for several reasons. First, it can be considered a hard currency, thus (nearly) everyone takes it. Second, in case of emergency you have something left in order to get out of harms way (fast transport, secure place, good doctor, etc.). Third, peace of mind, I never ever used it, so far.
Additional small backpack/rucksack/handbag
I use this for several things. When travelling on the bus, the backpack is stored in the “cargo bay”, thus I put my food, books, etc. in my small rucksack. While exploring a city, I use it to put in my camera, food and a map. Also it is great for shopping. There are many more situations, where you don’t want to run around with your big backpack or where it is inaccessible.
For safety, security and peace of mind. One of the first thing I do in a hostel is: I get a locker, put all valuable stuff in it and lock it. I recommend having a small, medium and a big padlock. Since every hostel has different lockers. Also you can lock your backpack or other equipment too, if you like, I don’t. They are cheap, small and extremely practical.
First aid kit
A first aid kit while travelling can save your day. Its mostly to treat inconveniences and slight “injuries” fast and effective. Some patches, some stuff for your digestion and an anti-infective agent. Check my package list for safety and convenience for hints about other things you should put in.
Extra food and some water
Especially on long trips you should always have some extra food and water with you. The bus could break down, a road block could force a delay, etc. Also you can share it with fellow backpackers and/or natives.
Something to entertain you
This could be anything a live-journal, a book, a MacBook, a mp3player etc. While travelling you will wait for transport, being in transit or sometimes you just want to have a “break” from all the adventures and cultures. Then you want something familiar from “home” or just something that helps you to relax. I stick to books and mp3s. In case, it is something expensive: be aware that you can use it appropriately – I would not get out a MacBook in a Bolivian bus, cause it could be the value of all other passengers yearly salary summed up…
If you are looking for good book suggestions check out Books to Read while Traveling from The Art of Backpacking.