I have done this in less than a week: i have beat ALL the levels AND got all the treasure chests. i also finished the ghost medal and the secret map and i have all of the spray paints. after u get all the chests, it says 100% complete and i think some other sentences that u should read. here are the unlockable stuff:

secret area: get all
peices of secret map

ghost mini boss: get all
peices of ghost medal

triple star: go to the end of secret area or defeat the shadow guy of team squeak before the shadow star. when u kill the shadow guy from team squeak, the triple star rod will apear.

fire, ice and spark swords: get sword and mix with fire, ice or spark. fire + sword = fire sword. ice + sword =
ice sword. spark + sword = thunder sword.

ice and spark bombs: mix bomb with ice or spark. be careful. there is NO fire bomb. trust me.

ghost kirby without ghost boss: mix 2 sleep kirbys together and it will make ghost kirby.

undefeatable kirby: get a thing that will make u undefeatable and u will be blinking yellow and white.

ok. i am done with these things. i hope these things are usful. i tested every power mix. it obviously worked. my hands are tired. talk to ya later!