White tail deer musk in the woods - you smell it where there they had been "marking". It gets your attention.
Sometimes, a rotting animal in the ditch hidden from a roadside view. Yuck.
There's a 3-mile perfect loop that I ride for fitness at night and in the winter (near my hoouse), where I can choose the number of times around for a particular workout. It passes through a neighborhood where I simetimes smell marijuana smoke.
Several of our Potomac River parks share access with undergound sewage and storm drain lines. That smell isn't so pleasant.