This happy little robot is designed to enable my students to mount their Raspberry Pi, connect up and program away. The little stepper motors don't drive him very fast, but he is very reliable and the battery life is impressive! The parts for this bot are easily available on eBay for under $50 (Australian)

Designed for something as small as the Up! Mini.

I printed mine on an Up! Mini with supports in two prints (9hours for the body and 3.5 hours for the wheels and sled). Carefully calibrate the print bed and warm the room because the body is prone to warping.

I am currently working on a castor wheel for the back and new wheels that have bigger diameter but less volume.

You will need:

A Raspberry Pi Model A+ or B+ (Preferably running Wi-Fi)

2 x 28BYJ-48 stepper motors (with ULN2003 Driver test Module Board)

1 x HC-SR04 Ultrasonic Sensor

1 x PowerBank Portable USB Charger battery (http://www.ebay.com.au/itm/2600mAh-External-PowerBank-Portable-USB-Battery-Charger-iPhone-4S-5-Samsung-S4-/131405859536?pt=AU_MobilePhoneAccessories&hash=item1e98664ad0)

1 x Adafruit Perma Proto Bread Board (1/4 size)

4 x 5mm LEDs

A bunch of female - female dupont jumper cables

Some 2mm stainless steel pins

A whole bunch of hook-up wire

Some header pins

Some rubber bands will give the wheels a bit of grip.

A some solder and a whole bunch a patience (the cables only just fit inside)

Some helpful tutorials for programming the Raspberry Pi are:
LED: https://www.youtube.com/watch?v=J9cEAGTLaC8
Stepper Motor: https://www.youtube.com/watch?v=Dc16mKFA7Fo (I found his wait time for the stepper motor needed playing with)
Ultrasonic: https://www.youtube.com/watch?v=xACy8l3LsXI

UPDATE:
I have found that running the stepper motor controllers of their own 9V battery (which fits nicely under the pi) I get better torque and speed.