I am looking for challenges to do on my spare time in forza horizon 4, preferably offline. I do not have any challenges to offer, but please comment on this post if you have any ideas.
This is what a couple friends and I are doing. It starts mostly at Post #13. It’s not “online” but ya need the Lego pack and the ability to run rivals. But no public lobby needed.
Thanks! I am trying the classic muscle a class races in LEGO.
Being first on every PR stunt. Being first in every rival category. Being first in every story. It could take about few years to complete 
Another challenge is to build not so easy car and race with it. Or take directly undrivable stock car and race with it.
You can race 50 laps Goliath or some other endurance stuff.
You can win all races on unbeatable AI with stock car.
… it’s a sandbox game, there is much more challenges than time.
I made a challenge for the Formula Drift Viper to win the Goliath on Unbeatable with no rewinds. I even made a Vinyl for it on my HUB.
Make the fastest car you can on a budget, say 50,000 CR for both the car and all tuning parts. Run Rivals and see how high up you can get.
Also, I very much enjoy tuning cars so I frequently grab a random unused car in my garage and see what I can do with it.