I swear by jogging. It tones your body all over if you're doing it right. Preferably jog outdoors as the uneven terrain is more challenging. Work at a slow, steady pace and remember to use your arms so you'll be stretching and toning your upper body as well.
Swimming is good too, but you will need to do breast stroke for a better workout. Can become quite tedious though as you'll need to do a lot of laps to tone up!I
If you find exercise a bit boring, then I suggest you do what I do and vary by jogging 2 evenings a week, and another 2 evenings do sit-ups/crunches at home. The home workout is quicker so you only need to allow 20-30 minutes, but do ensure you stretch your stomach muscles after your crunches.
Enjoy!