You can try beef or sausage meat, and slice it into thin strips so it mixes in with the noodles easily. Those imitation crab sticks are nice if you don't mind processed food. And for a vegan option, carrot strips and beanshoots works well. I think any ingredients that can be cut into thin strips and not too heavy or moist (hence cucumber not a good idea) will be ok.
Hope that helps!
Thank you for the suggestions! Now I'm hungry xD Looking forward to buying some noodles :D