In a recent study published in the Journal of Animal Science, researchers described that just adding water to the cats' food, and feeing multiple small meals each day instead of two meals or free-feeding, resulted in increased physical activity.
So if you can manage to feed four meals per day (remember, don't increase total daily amount of food fed; just break it down into smaller meals) and add water to their dry food, you may see a happier, healthier, more active cat.
P. Deng, E. Iwazaki†, S. A. Suchy, M. R. Pallotto and K. S. Swanson; Effects of feeding frequency and dietary water content on voluntary physical activity in healthy adult cats. J ANIM SCI March 2014 vol. 92 no. 3 1271-1277. doi: 10.2527/jas.2013-7235