To use the following conversion cheats, head to the game’s title screen and press Up, Right, Down and Left on the D-Pad. This should bring up a cheats menu. Insert the cheats below for the desired result.

• Flower Power - Converts explosion effects to flower explosions
• Maestro - Converts base blocks to jingle blocks

Addition by Thiago: It appears the game doesn't save your progress when using the above cheats.

To unlock additional challenges, obtain silver and gold medals on currently-available challenges. By obtaining at least a silver medal on all challenges, you will unlock expert-level challenges, while attaining gold or higher will net you master-level challenges.