A pretty nice GP150cc challenge has been suggested on the french MB a few years ago, and it's still running.
Rules: get the highest amount of coins you can on MC1 and win the race. You HAVE TO finish the race in 1st place, otherwise your record CANNOT count. The amount of coins you got when the race is over is WHAT REALLY MATTERS, not the chrono itself evidently. If there is a tie between several players (same amount of coins), the player with the fastest race wins though. The challenge must be done in 1-player mode.
NB: there are 40 coins on that track + 12 items (virtually 24 coins).
Here is the current PAL ranking on the french MB so far:
1st -- Florent Lecoanet - 50 coins - 1'19"71 - DK Jr
2nd -- Camille Batier - 46 coins - 1'11"65 - Bowser
3rd -- Pierre L'Hoëst - 43 coins - 1'19"87 - Koopa
4rd -- Wabs - 41 coins - 1'19"79 - Koopa
5th -- Samuel Gomez - 40 coins - 1'23"73'' - Luigi
6th-- Christophe Paquin - 39 coins - 1'13"xx - Bowser
7th -- Martin Ricci - 39 coins - 1'19"40 - Koopa
8th -- Johan - 37 coins - ?'??"?? - ?
Feel free to add your name and your record

Good luck everyone

My personal record (50 coins):