Depends what you want? By "general fitness" you might well mean areobic endurance. For this anything that raises the heart/breathing rate and can be done for over 20 min, such as swimming, running, hill walking, dancing, horizontal dancing (ok maybe less than 20 min).
As someone said, Aikido (particualry ukemi) should keep you pretty fit!
For muscle definition I can't help ya, not a body builder, a progressive weights regime I guess, though I'm not so sure that'd be good for your Aikido. Maybe a dictionary
On a personal note I think you can exercise every muscle in your body, just using your own body. Gyms are a waste of time. Nature provides