Depends on your purpose.

You can do exercises that hit other parts of your body that bicycling doesn't help much. Crunches and weight training are good here.

Now if you are talking about enhancing bicycle performance that is another story.

Generally, anything that hits your cardiovascular system will enhance your cycling especially if the legs are involved. Cycling is better but if you can't cycle every day and you hate trainers, something is better than nothing. Stair climbing, running, jogging, walking, rowing, cross country skiing, aerobics, etc.