I'm not sure how long your tube is, but maybe you could use your remaining yarn (without adding any new yarn) to thread through the loops on your needles and draw the end closed. Then stuff the tube with beans or batting (or both), close the other end, and use it for a draft stopper in front of a door or window.
Then if you're still hung up on knitting a tube, why not do one of the Dr. Who scarves
? That should keep you going round and round for a long long LONG time.