There's a product-category of puzzle feeders, which provide a number of benefits — they slow down eating, provide enrichment, and somewhat increase activity.
Move the food bowl to different locations in the house, such as upstairs or downstairs and rotate it so that the cat always has to walk to get to its food bowl.
Move the food bowl as far away from your cat's favorite haunts as possible. Many overweight cats will sleep and lounge around near the food bowl so they do not have to go far to get a snack.
You can also throw their food to them to make them chase it at meal times.