Customizable Line Following Tracks you can print yourself
You build the track from as many 20x20cm tiles as you like. You can choose from standard black line patterns such as straight paths, corners or crossings, but you can also select several colorful tiles if you have a color sensor. Find the full set of tiles here.
Some of the tiles may provide some real challenges, such as Tile 19 and 20. I haven't tested these myself yet, but I'm sure someone will come up with a solution. Feel free to use these tiles in competitions as well. Just let us know which cool tracks you come up with!