If you really want to push your body to its limit, try a Bikram yoga (or any hot yoga) class. I was in pretty good shape when I started and it took me almost 6 months before I could even attempt to complete all the asanas (and about the first 10 classes I felt like I came close to passing out a few times). I've been practicing now for 6 years and still have a long way to go before I feel I'll have mastered the asanas. It easily meets or exceeds the impact that weightlifting has on my body.