Ride to work kit

I regularly, if not that frequently, ride to work (12 miles each way). It's a good work out and has been really valuable over the past three years of recovering from illness and injury.

This is the stuff I always take with me:

Pollution mask
Lights
GPS
Heart rate monitor strap

I ride to a heart rate now (up to rather than above) rather than a speed. It stops me from overdoing it.