Yoga is a great idea...

I swear by the Yoga Zone series and have 3 DVDs that I use frequently:
- Introduction to Yoga,
- Flexibility and Tone, and
- Conditioning and Stress Release

All three are lead by Alan Finger (he's a phenominal instructor) and have 2 women doing the poses: one woman demonstrates the ideal form and one does the poses in a modified form for those of us who are less flexible.

While all three are rated 'beginner', they're still challenging after using them 1-3 times a week about 2 years now. Each DVD is significantly different so you get a different result with each. They're pretty well known, so you'll find them on amazon and in your local Borders or Barnes & Noble...

Hope this helps!