Would be nice if the Coin Race mode had other cars in it. Also at least one race could use an additional checkpoint gate, as is you can skip some of the "track" by going an alternate route.
Tips: I make money fastest doing sprint races - just pick a short one and keep repeating it as fast as possible. Some races have shortcuts - you can skip portions of "track" by finding alternate routes to the next gate. For a change of pace you can run a track backwards to get crash $$ instead of winning, or do a coin race and keep stacking mines on one spot (you have unlimited) until they explode and send you flying.