From Muppet Wiki

Scooter is depressed because Skeeter is good at yo-yo tricks. Skeeter brings her yo-yo to school and dazzles everyone with her amazing skill. But the tables are turned when Skeeter can't come up with a project for the science fair, and she has to ask her brother for help.

Piggy's science fair project is a "garbage garden". There are no further details available on what a "garbage garden" is.