My children liked to have lego base plates with crossing, roads, T-joints and a place to build a house.
This is a remix from http://www.thingiverse.com/thing:615256 which was a remix from http://www.thingiverse.com/thing:5699
Added an extra part in the openscad file to make an extra nop free row. To make a curve, crossing or T-joint.
STL files for several 20x20 groundplates provided
Printed with 0.2mm layer heigth.