Old Big Oak Flat road used to be the access road to the valley until it was deemed to be too unsafe and eventually destroyed by rockfall. Today it is an interesting hike (partially on the old pavement, partially scrambling and route finding over rock fall) with great views of the Valley sights.
This is not a maintained trail - be aware that rockfall, dead trees, and other hazards will block the way from time to time. This guide covers the 'easy' first portion of the road from Yosemite Valley. It extends further up the Valley walls but requires additional route finding and rock scrambling skills.