A head-mounted device may have optical modules that present images to a user's eyes. Each optical module may have a lens barrel with a display and a lens that presents an image from the display to a corresponding eye box. To accommodate users with different interpupillary distances, the optical modules may be slidably coupled to guide members such as guide rods. Actuators may slide the optical modules towards or away from each other along the guide rods. The guide rods may be formed from fiber-composite tubes with end caps that are fastened to a frame in the head-mounted device. The tubes may be partly or completely filled with cores to add strength. Low-friction coatings such as metal coatings may be formed on the fiber-composite tubes and the corresponding inner surfaces of the optical module structures that slidably engage the fiber-composite tubes.