I thought this was a neat article (and study) that explained when to use active, so moving around, or passive, standing rest, after an interval. The basic findings were if an interval is short, use passive recovery. If the interval is long in duration then use active recovery.
How can you apply this to your training?
Swim: Use active recovery when swimming hard repeats of 200m or longer distance intervals
Bike: Use active recovery for climbs or intervals lasting 3K (5 minutes) or longer
Run: Use active recovery for 800m or longer distance repeats