Here's a good rest day workout for people with access to a treadmill, it's only 30 minutes long but should get your heart pumping.
Start off with a 5 minute warm up walk, choose the pace wisely as this is the slowest you will go, get the incline to 1 degree.
After 5 minutes, keep the same pace but raise the bed to 4 degrees.
Here is where you can do one of two things:
1. Raise the bed by 1 degree every minute or
2, Raise the bed by 2 degrees every 2 minutes (my preferred method)
Maintain the same pace.
Keep raising the bed using one of the methods above until you get to the point where it is really hard going.
Either way, when you get to 20 - 22 minutes start dropping the bed, aim for a reasonably easy drop back to 4 degrees by the time you get to 25 minutes.
Drop one degree a minute until you reach 30 minutes.
As I've now maxed out my incline (20 degrees) I've started to increase my speed, spreading a 1 kph increase through the 30 minutes but mainly starting once I hit 14 degrees.
It's a good fat burner and adds to your daily step count and a low impact exercise.
Happy running (or walking in this case).