Inline Skating / Roller Blading
Inline skating or roller blading is a great way to enjoy training outside if you have an area and climate conducive to this type of training. Skating on a set of roller blades is substantially less jarring than running. Additionally, the forceful push outward is a tremendous way to strengthen the gluteals, which helps with many running injuries.
Using a heart rate monitor may help you maintain your target training level.
Research conducted at the University of Wisconsin compared heart rate and oxygen use during inline skating and treadmill running. They concluded that the two activities had similar heart rate and oxygen use patterns.
Remember to wear appropriate safety gear (helmet, wrist and knee pads). Unpredictable tumbles can ruin the day.