Easy one to do, Race along the motorway on Forza Horizon 3 and drive close to all the cars you pass, Do this 50 times in a Extreme Track Toy Car.
You can use some of the following cars:
2014 BAC Mono
2013 Caterham Superlight R500
2009 Lotus 2-Eleven
2013 KTM X-Box R
There are many more cars to choose from this is purely to be used as an option to enable the completion of the challenge.