-Completely upgrade Toa Nuparu at the Matoran Enclave shop, then collect 500,000 Lego pieces and buy "Bonus Level 1". Go to the Piraka Playground. Go to the other side of the playground. Once there you should find a cave/pathway between the Piraka VIP lounge and the Piraka diving board. Go inside the cave to find three portals. Go through the first portal on your left. Once you are in, switch to Toa Nuparu and start destroying several enemies while collecting Lego pieces. Do this quickly, as you have a 112 second time limit. You can get an average of 50,000 to 70,000 pieces every time you try the bonus level.
-Collect all the Lego bits in the Matoron Enclave then go to the Priaka Playground. Go back out. Repeat this for tons of Lego bits.