Cats were born to hunt, so hunting for their snack can encourage activity during the day. Utilizing their normal amount of kibble (making sure not to add additional calories) but placing small quantities in different locations through out the house. If you have a cat tree, placing kibble on higher levels encourages jumping.
Interaction and play with your cat builds a bond and keeps them moving. Laser pointers and feather toys work well, along with toys they can utilize individually such as turbo chasers. Something that grabs their interest and encourages interaction.
If you have stairs in your home encouraging them to go up and down can help to burn excess calories. Encourage them to follow you up and down while you're doing housework. Keep litter boxes in one location or even a different level and food on another.
Some kitties can adapt very well to outdoor time on a harness. This can provide exercise and stimulation while still keeping them safe and with you. Keep in mind, that for some of the more timid at heart, outdoor time would be too stressful and focusing on indoor activity is best.
As with anything new, make sure your kitty isn't dealing with any health concerns before altering their routine. An evaluation with your veterinarian can help to rule out arthritic changes or other health concerns that may be contributing to inactivity. Your veterinarian can also give you a target weight for your cat so that everyone is on the healthy track to activity and weight management.