I had a similar problem, I needed a way to encrypt data on USB sticks in a portable way.
First I thought on buying one of these encrypted USB sticks. There are two types: some rely on a software app, while others have a physical padlock. I discarded software option because these were Windows-only, and because I can't be 100% sure the software is secure and cannot be hacked, or comes with a backdoor, or even "phones home". I also discarded the padlock ones, since some users reported data loss and needed to format the drive from time to time. Also I can`t rely on the padlock not being broken with use.
So I'm currently using TrueCrypt to have encrypted container-file-based volumes on regular USB sticks. But this is not a fully portable solution, since TrueCrypt needs admin permissions (thus if you go to an Internet cafe, or try to use a public terminal it won't work). On the other hand, these volumes can be configured to mount automatically (which is a good thing but only in case the computer is secured), and it works on both Windows and Linux, so I'm using it to move data from home to work. Also TrueCrypt is Open Source, so you can inspect the code and make sure it does just what it says.
To encrypt account passwords, I'd recommend using a more standard file format (like zip) that also supports encryption. A good example of this is 7-zip that supports AES-256 and is also Open Source. The pros: you can carry a portable 7-zip version in the USB along with the encrypted files, and you could run it in any computer without caring about permissions (so it is a fully portable approach). The cons: every time you need to add or modify a file in the zip container, you need to decrypt the container file, modify it and encrypt everything again. As you can see, unless you remember to file-shred the temporary unencrypted zip, it could be recovered by anyone. So yes, it is a good approach but for read-only files, provided you know what you are doing.
In the future, I'd probably code my own dropbox-like solution, with both desktop and mobile versions.