I try to keep my training simple, You don't need to worry too much about changing it up..repitition build muscle memory. If I am somewhere that I can't do Muay Thai at a gym, I try to go back to basics...skipping/shadow boxing. I like to mix them up by skipping for 5 minutes, then shadow boxing for 5 minutes. You can also throw some push-ups/squats/sit-ups into the cycle if you want. The skipping and exersizes are just to keep in shape and help harded your muscles. The shadow boxing is to help tighten your technique, and is the most important to improving you skill.
some thing to remember about shadow boxing...
1)throw proper, fully extended punches and kicks just like you were sparring...half assing it doesn't improve anything
2)Keep you hands up at all times, and don't forget about your defensive techniques (push kicks, leg checks, step back's, lean back's, pivots)
3) throw realistic, simple combinations of punches/elbows/kicks/knees...I like to practice combinations that close the distance ie. push kick-jab-hook-elbow-knee or combinations that change levels ie. jab-cross-low kick-high kick
4) Think about every movement, and make your technique as clean as possible. Proper technique while shadow boxing will help to develope muscle memory even though you aren't hitting anything.
I hope this helps