In many of the robots, we would use the LEGO Steel Ball so you should try to acquire one. Even better if you have two of those. One is available in the LEGO Education EV3 core set 45544, but the LEGO EV3 Retail Version 31313 does not have them.
Where to buy them from?
You can buy them, or you could really be creative when replacing them with other third wheels. It is your choice. If you decide to buy them there are a number of places. The set is called "LME Replacement Pack 3 2000702" so a quick internet search should find you some good placed to buy them from. Generally, LEGO should have them on their site. Below you can find a picture of the Steel Ball and here are a few links that you could use
Building instructions for a LEGO Mindstorms EV3 robot where the motors are placed in an unusual manner. They are vertically placed on the sides. This robot construction base is very convenient for a quick and interesting robot on which you could build.
This is just basic programming where you control the robot with both motors. Make the robot move in Circle. When done mark the task as completed.
Program the robot to move forward at a large speed and then suddenly stop. Is the robot falling on its face? If yes, improve the construction so that it does not fall. If no, change the construction so that it falls.
What other parts do you have at home? Try to add a head and a few - 3-4-5 hands to this robot. Probably you could use the touch sensors. Experiment. Take a picture and show it to us.