This purple plant is Bugleweed (botanical name: Ajuga reptans), also commonly known as carpet bugle or simply ajuga. While it's often grown in gardens as a hardy groundcover, it frequently escapes into lawns and fields, which is why you'll spot it growing wild in grassy areas this time of year.
Turns out what we thought was a beaver's home at the edge of the East Oshawa Creek, is actually the residence of a raccoon.