For me I have rest days from running. On those days I will do low impact and usually low intensity exercise, mainly in the form of rowing, but occassionally spinning or swimming. The spinning is usually low gear, with similar cadence to running to give you an idea for cycling. Active recovery suits me. I also do a little bit of strength training.
Yesterday I didn't follow my rule, and had an intense rowing session for approx 60 mins. Today my running interval session was complete pants - too tired. So if the priority is running, cross train at an intensity that won't spoil the running sessions. I have targets for running and rowing, so it is a bit of a balancing act.