|wow... I just found out if you are close to a key, then breaking blocks will make random colored coins appear in the color of the key you are close to, as a hint. that's clever!|
Also there are certain blocks that are only removed after you complete the golf minigame of that level... coins are required to play golf. That's pretty much the only reason for collecting coins.
got the ground pound ability from W2 The Volcano's Base... these levels are really starting to open up more now. You can actually get lost on your way to finding the grey treasure.
W3 The Pool of Rain and W4 A Town in Chaos - found the two gear treasures, which opened up the path to the South section
S1 The Grasslands - wow this level is annoying... unseeable pits leading you into a water section where you have to backtrack... found a music scroll that lowered the bridge, leading to
S2 The Big Bridge - very quick grey treasure, just gotta get passed the jumping fish - found a beanstalk seed... that affects 3 different levels I've been to already.
Some of the changes lead to red or green treasures, others just lead to music coins. It's literally pointless to worry about collecting the music coins at this point, because you have to collect all 8 in a single playthrough of a level to get perfect on it. That's gonna be like, an extra post-game challenge after I've collected every treasure.
N3 Vast Plain - found a path to a green treasure... past a platforming section with a moon that shoots stars at you, and another golf door.
no idea where to go now... temple guide time... wait so apparently there IS something. ah.. an emu glitch prevented me from climbing all the way up the vine... it could be solved easily just by manual saving and continuing from title screen.
2 skull ring treasures opened up
S3 Tower of Revival - fire run and easy puzzle... got the flippers omg yes finally!