Cardio doesn't tone. In fact, cardio can be detrimental to toning if you do too much, because endurance exercise burns lean muscle tissue and fat.
Toning is an inaccurate term. When you tone, you're actually building muscle. When the muscle becomes larger, it stands out more, gives that body part more shape. That's 'toning'.
You want to do high-rep, light weight resistance exercises like Chris said, and don't hate on machines, they're great for getting that extra few reps on isolation movements.
To keep muscle mass while doing intense cardio, use interval sprints. Warm up to a good steady pace, and then go balls out for 60-90 seconds, then go back to your cruising pace for 2-3 minutes, then sprint again, and so on. Do three to five of these patterns in a 20 minute cardio session. And always do your cardio post-lifting so your body will already be in a fat-burning state.