The Raspberry Pi is a great little device for portability, and it's even better when it's combined with a touchscreen and a lithium battery. Adafruit shows off how to build your own.
You'll need access to a 3D printer for the case, but building your own little portable Pi is easy and inexpensive otherwise. This one uses the Model A+, so the form factor's super-tiny and portable. Head over to Adafruit for the full guide.
Touch Pi: Portable Raspberry Pi [Adafruit]