Ideas for strengthening a steel tube?

Have a 3” O.D. x 7 ft steel tube, wall thickness about 1/16” and I need to increase its ability to withstand lateral loading. Can’t attach anything to the outside; only the inside. Best thing I can come up with is to fill with concrete, but I’m not excited about the added weight that will introduce. Are there any other lightweight, relatively inexpensive fillers out there? Or other thoughts?