One of the project ideas I came across on the BIE website. The exploration of biomes works well with our ecosystems unit.

http://www.mrsoshouse.com/pbl/pblin.html

This site provides a long list of PBL projects, problems, and activities, as well as informational pages wha PBL is and how to create your own PBL activities. A range of subjects are addressed on this site and many projects have leveled options.