This site, Universal Leonardo, is a beautiful way to explore the many, many accomplishments of Leonardo da Vinci. The site was originally set up as an adjunct to a museum exhibition, which is now closed. You can explore his paintings, drawings, and his writings. One of the features I liked best was the magnifier that appears with his drawings and paintings. With it, you can more closely examine his work.
This would be a terrific way to introduce students to the breadth and depth of Leonardo's works and curiosity. It would also be a good place to send students as they are studying the Italian Renaissance as it not only has works by da Vinci, but some of his peers as well.