I'm sure that I was the only one to discover these secrets for the DS remake.

[Mario Sports Mix]

1: Yoshi can defeat Fire Chomps by eating them.
2: Yoshi can burn boxes and signposts with his Power Flower. They disappear seconds later after being set on fire, but respawn later. Burning signposts can't be read.
3: It's been said that in Big Boo's Haunt, Wario can't get Star 5 (the one where you fight Big Boo for the final time). He can get the star by backflipping off the ? Block.
4: Wario can defeat Big Steelys by punching them.

I added the first, third and fourth secrets to the Mario Wiki (as Mario Super Sluggers).

 

[MQJinx]

Also, Luigi can backflip over the water into a patch of grass and scuttle to the top of the castle before all 150 stars are collected. I used that technique to catch the rabbit on top of the castle.

 

[Mario Sports Mix]

Also, Luigi can backflip over the water into a patch of grass and scuttle to the top of the castle before all 150 stars are collected. I used that technique to catch the rabbit on top of the castle.

Found out about that as well from a video.