I wanted something my 3 y.o. and I could build with, but didn't find anything that was easy to use yet allowed building of large structures.  Now he is super excited whenever “3d printer wakes up and makes more blocks”!  

These can all be printed without support and with reduced settings to save weight and material.  They are still quite strong and robust with reduced settings, and look great with fancy filament!

Settings I use:

• 0.4mm nozzle
• 0.2mm layer height
• 1 perimeter
• 3 bottom layers
• 4 top layers
• 10% infill, triangles or grid

IMPORTANT: for parts that have pins, be sure to add a modifier that sets the pins to 100% infill.  Or grab one of the pre-sliced .gcode files where this is already done!