Since we're throwing out kinds of cardio...
A friend of mine plays water polo. Sounds posh and effiminate, right? Well, it may do, but according to my friend (who also trains MMA) it is an amazing workout.
In waterpolo you are basically treading water the whole time, but with a lot of your body out of the water, keeping a keep high level of cardio. To get the ball off someone, you can basically do anything to get them down into the water and keep them there, until they give up the ball. So it's basically wrestling in the water, and is very intense. Being in the water makes you very flexible as well.
I think I heard Joe Rogan say that BJ Penn used to play water polo before he got into BJJ.