A genuinely different way to experience Stonehenge — free, quiet, and on foot rather than queuing at the visitor centre. Worth downloading one of the National Trust's self-guided walk leaflets to actually find the Avenue and Cursus, since neither is obvious without a map.
The chalk downland surrounding Stonehenge itself, owned and managed by the National Trust and free to walk at any time — the stone circle behind its fence is a paid English Heritage attraction, but the wider landscape around it is open access. Self-guided walks take in the Avenue (a 2.5km bank-and-ditch earthwork thought to be the ceremonial approach to the circle), the Neolithic Cursus monument built centuries before Stonehenge itself, and King Barrow Ridge, with dogs welcome throughout.
National Trust land — usually free to walk; some sites charge for parking.