These robots starting smiling when walking down the road by them selves :)
They do not require any power, motors or what so ever. They just need to be placed on a small ramp. These toys where quite famous in the 1960s and it's still fun to play with. The walking mechanism is described by this old patent, from John E. Wilson, 1938.

Step 1: Create Robot & Ramp

The robot:

Download the STL file and print the 3 parts. In this case, a piece of 2.85 filament is necessary for acting as hing point between the legs and the body. But could be something else too. (Just bore the holes bigger if you need to, or print at different scale.)

Assembling the robot is quite delicate:

-First remove the brim on the feet (or print w/o brim), so the surface is as round as possible.

-It's very important to mount the legs proper into the body. The shaft (filament or something else), should be solid connected in the body.

-The legs should freely rotate around this shaft in walking direction and should not rotate/move in other directions. (If necessary, drill out the holes in the legs making them loose. If you hold your robot horizontal, the legs should fall downwards, when you flip the robot, the legs should again fall downwards.)

The ramp:

There are different options, use plywood for instance. The surface needs to be flat. Putting a sticker on top, will already make the robot stop walking. Play a little for finding the right angle for the ramp. An angle between 10 & 15 degrees will probably work.

Start walking:

Best way to start your robot walking: give it a gentle push side wards. The legs will do the rest.

Speeding up your walking robot:

-Adding some mass.

-Play with different angels & roughness of your walking path.