well ben, i would do a little of everything.
1) jump rope, very much needed and is good for endurance.
2) swimming, there is no other cardio that works you like swimming. it is a known fact.
3) running, build up your speed and distance. it is good for a normal cardio workout. it is very good to get repetetive and build/ use good breathing techniques.
4) roll/ fight/ spar, this is very good too.just messing around, going light with a partner. Working submissions only and grappling is very good for a cardio workout. Work subs, and go light. it is not a fight so you can also work on perfecting moves and teqhniques.
just a few i would try bro