I have found the best workout
Equipment - 40 lb+ weight vest & 35 lb+ kettlebell
Basically you just do a kettlebell routine while wearing the weight vest for 40 - 60 minutes. This just knocks me on my ass and from what I am reading about it it puts heart rate way into the redline zone.
I will hammer away at this now - I love it.
Same as this but wearing a weight vest