A few years back at AgQuip retailers were flogging $400+ sensors for farm driveways to detect and notify property owners of activity across certain points. Thse days $400 can buy you a lot more automation.
Watching NYCCNC Youtube channel video that involved lasers to count things gave me an idea. Why not use the same near invisible low powered lasers to create an invisible beam across many areas like a driveway or back door. An Ardrino can easily poll these lasers well over 1 million times a second. Using two laser beams a few centimetres apart would allow you to predict the number, direction and potentially the size of the object(s) passing through. If different height lasers (with a magnetic sensor) were setup you could predict animals (kangaroos, echidnas) kids, adults, cars or trucks.
Once an object has been triggered why not activate a $30 raspberry pi camera or another camera and start capturing and uploading offsite. Notification triggers could be sent (SMS, Tweets, Emails etc via a central Raspberry Pi computer).
Parts would be inexpensive and trivial to setup.