Also you might check out How Everything Works by Bloomfield, and the show Mythbusters.

Or if you have a more practical bent, maybe something on bridge collapse? If you are good with motors already, electromagnets can be fun.

if u want an easy one electromagnets can be real real fun for sure as already said.

but try making something more practical. a working model of something that u think u could do. make something related to hydraulic cranes as they have enabled to pick up great weights. recently i watched a discovery showcase show show in which they showed a hydraulic crane picking up a newly built bridge to place it across a river. it was amazing. the bridge was built somewhere else before it was to be placed over the river. it was an amazing show. i think making a project on hydraulic systems would be the best thing to do.