There are 5 Go-Kart races at the Carnival. But when you beat those, that opens up 3 more city races.
I don't know if the Red Ninja suit does anything. I never even tried on the Green one. But I do know that the Black one makes you practically invisible to authority figures, even in the daytime, outside of doing something major like hitting someone. Basically, you can Trespass and Break Curfew without getting in trouble.
Consumo only took me about 2 or 3 tries but the high-score winning try took me like 5-10 minutes. Or at least seemed like it.
And as far as I know, I don't know if you can do any tricks on the skateboard aside from spinning. You can usually do a 360 spin if you're going fast enough and hold the jump button down. You can get a 540 if you jump over the stairs leading to the football field, maybe a 720 if you do it fast enough.