Joy Ride Turbo

OK - for this achievement you just need to have collected over one million coins in total across the whole game. This can be on race tracks, stunt parks or as winnings.

On race tracks & stunt parks the small gold coins are worth ten each & the larger blue starred gold coins (which replace the car part crates, once collected) are worth 100 each.

As a bonus in the stunt parks the smaller coins will come in lines of five. Collect all of these & another row will spawn nearby. Collect all the rows in a sequence (it could be anywhere between two & ten groups of five - maybe more) & you get a 250 coin bonus.

The quickest (but poss'ly most boring) way to get the million once you've finished with career is to grind the 300 HP Grand Prix in Championship Series.
You get 130 thousand for a win.

The one million does not have to all be in the bank @ the same time so feel free to continue to buy cars while collecting.

The other solution is great but I was playing through this 10 year old game and needed more coins to complete it and came up with what I think is a good route for farming coins in Crimson Park (Stunt Park). This route, assuming you have the fastest car unlocked (Inception Sport) and you have collected all the car parts (they are replaced on the map as a circle with a star that gives you 500 coins. Each regular coin collected in stunt park gives you 10 coins. The best way to get the most coins is to collect them in the string of 5, you will hear a ding when you collect each string of 5 and then another string of 5 will appear ahead of you. Get the strings of 5 until a bunch of coins shower you which is a bonus of 1,000 coins.

See the video attached. I run the route twice and get 6,100 coins (includes two star coins 2 x 500) the first lap and 5,100 coins in the second lap. Each lap takes about a minute.