The Raspberry Pi is a great little computer for all kinds of projects, but it's not particularly portable. Instructables user agent036 decided the best solution was to stuff everything inside a lunch box.
Agent036 uses a handful of accessories beyond what the Pi usually needs, including a USB battery, a case, a small 5" LCD, a Bluetooth keyboard, another set of rechargeable batteries and a Wi-Fi dongle. The end result is a portable little machine that should cost well under $200 (lunch box not included).
Lunch Box Computer with Raspberry Pi [Instructables]