Skipping rope is also a great option. Cardio is up there with running and it's not as hard on the knees. We usually start every session at my muay thai gym with

Skipping rope is also a great option. Cardio is up there with running and it's not as hard on the knees. We usually start every session at my muay thai gym with it, and whenever I travel I'll just throw a cheap rope in my bag. I do love just getting out and running though!