Okay, so you don't have much space. You'd really like to build a line follower or micromouse robot, but there just isn't room in your apartment. Besides, building a test course for those robots is really involved and takes up even more space. Or does it?
So, why not build a line follower robot that is so small that the test track can be as small as a regular sheet of paper, and you print the test track on the paper using your computer printer or copy machine?
Fourth law of Robotics: Great designs spring from little ideas.
ELM-Chan must have the same space problem because he (she?) designed and built a really neat little (really little) line follower robot that will drive in circles around your coffee cup, or make loops and figure-8s while staying within the bounds of an A4 sheet of paper (or probably 8.5 x 11 inch).
The design is really creative and incorporates a couple of tiny vibration motors (the kind you would find in your cell phone - or perhaps your wife's cell phone - she'll never notice it's missing....); an ATmega8 controller (Atmel) six photo reflectors, and a couple of CR2032 lithium cells to power the whole thing. Fully loaded weight is around 15 grams (5.29 ounces). The robot is capable of ripping around your desktop at a lightening fast 53 centimeters per second (oval course.)
Pretty darn cool. The next logical step would be to build a micromouse that size or slightly smaller and have it run a mini-maze.
Of course, by now you know the way my mind works. I want to see a micro-Robo-One battle robot, or perhaps a fully functional micro-Roboraptor!