Crossfit. If I had to name one of the best adult decisions I've made thus far, starting crossfit would be at the top of the list. My only regret is that I sat on my ass for two years occasionally visiting the crossfit website thinking that I could never do it, but how awesome it would be to try.
It's amazing how my overall fitness has improved in just three months. During one of my first sessions I could only step onto a box instead of box jumping. Last week (after having not done them since) I started to do the step ups, but my coach told me to just jump. I was amazed at how easy it was when just three months ago I struggled so much. I think that's a true testament to crossfit's success. Even without doing that movement for so long I still improved immensely in that area because my overall fitness is so much greater.
As Kent said above, everything at Crossfit is scalable. You can scale push-ups until you have it down. If you can't lift the dumb bell, use a dowel until you have solid form and feel comfortable with the movement. I'm incredibly uncoordinated and consistently finish towards the end of every WOD, but once I got over myself and only measure my progress against my own previous performances I don't care that my name is in the last spot on the whiteboard. At least it's on there! A lot of crossfit boxes have trial classes for people to test it out. Find one near you that offers that option and see how it works for you.
It's not easy, but so so rewarding. In terms of quantifiable results, I haven't weighed myself in over a month and have never tested fat percentages or anything, but I am down a clothing size and a lot of my "softness" is gone. Even my abs are getting hard! I'm 5'1 and was 142 at last weigh-in so I clearly had some weight to lose (still do), but it's not about that. It's about being stronger, healthier, and frankly, much much happier.