To rack up Lego pieces, fully upgrade Toa Nuparu at the Matoran Enclave shop. Collect 500,000 Lego pieces and purchase the "Bonus Level 1". Go to the Piraka Playground and head to the other side of the playground. Look for a pathway that leads to a cave and enter the cave. Enter the first portal on the left. Switch to Toa Nuparu and start killing enemies while collecting the Lego pieces. You will get about 50,000 Lego pieces.