Runners: Cycling is the perfect complementary activity, because it maintains leg strength and cardio fitness while giving your legs a break from the pounding of running.
Cyclists: Running and rowing are both great options for active recovery because they can help to develop your quads, core and glutes.
Swimmers: Swimmers are notorious for avoiding any activity that doesn't involve ploughing up and down the pool, but if you want to take a break from endless laps, rowing builds strength in the shoulders, arms, legs and core, all of which can translate into better performance once you're back in the water.
Stretching: Flexibility workouts should be a part of your regular program, but are particularly important after a hard workout. Be sure to stretch as part of your cool down, later in day (before bed is a good time), and on 'recovery' days.
So next time that you're feeling sore from a long hard session, experiment with active recovery. Chances are you'll recover quicker and be ready sooner for that next tough workout.
References:
Ahmaidi S, Granier P, Taoutaou Z, Mercier J, Dubouchaud H, Prefaut C. Effects of active recovery on plasma lactate and anaerobic power following repeated intensive exercise. Med Sci Sports Exerc. 1996 Apr;28(4):450-6.
Choi. D., K. J. Cole, W.J. Fink, & D. L. Costill: Effect of passive and active recovery on the resynthesis of muscle glycogen. Medicine and Science in Sports and Exercise (1994).
Micklewright, D P. 1; Beneke, R FACSM 1; Gladwell, V 1; Sellens, M H. Blood Lactate Removal Using Combined Massage and Active Recovery. Medicine & Science in Sports & Exercise. 35(5) Supplement 1:S317, May 2003.
Sairyo K, Iwanaga K, Yoshida N, Mishiro T, Terai T, Sasa T, Ikata T. Effects of active recovery under a decreasing workload following intense muscular exercise on intramuscular energy metabolism. Int J Sports Med. 2003 Apr;24(3):179-82.
Signorile, J.F., C. Ingalls, and L.M. Tremblay. The effects of active and passive recovery on short-term, high-intensity power output. Can. J. Appl. Physiol. 18:31-42. 1993.
M Suzuki, T Umeda, S Nakaji, T Shimoyama, T Mashiko, and K Sugawara Effect of incorporating low intensity exercise into the recovery period after a rugby match. Br J Sports Med 2004 38: 436-440.
Thiriet P, Gozal D, Wouassi D, Oumarou T, Gelas H, Lacour JR.The effect of various recovery modalities on subsequent performance, in consecutive supramaximal exercise. J Sports Med Phys Fitness. 1993 Jun;33(2):118-29.