4 ideas
The expedition finds that the Ringworld's builders fell when a mold that consumed superconductors spread across the ring. It destroyed the power grid, transport, and the floating cities that depended on it. The survivors' descendants live in scattered pre-industrial cultures that worship the Engineers as gods and cannot maintain or even understand the structure keeping them alive.
The Puppeteers manipulate Earth's birthright lotteries for six generations, trying to breed humans for inherited luck, and produce Teela Brown. Her luck optimises outcomes for her alone. Companions suffer, events bend toward her needs, and the whole expedition may exist only because it leads her to her ideal life.
The Puppeteers treat extreme caution as sanity. They 'lead from behind', manipulating other species so their own members never face risk. Any Puppeteer brave enough to leave home and deal with aliens directly, like Nessus, is by definition considered mentally ill.
The Ringworld spins at about 770 miles per second to produce roughly one gravity, holds in its air with 1,000-mile rim walls, and uses orbiting shadow squares to create night. Readers worked out that a rigid ring around a star has no stable equilibrium and will drift into it. Niven accepted the correction and added attitude jets in the sequel, which shows hard SF working as a testable model.