The sacred city covers a wide area — easily 5 km from north to south of the monumental zone — so most visitors rent a bicycle from their guesthouse. Bikes are cheap (typically 500-1000 rupees a day), the roads through the ancient city are flat and paved, and the shade of the surrounding trees makes cycling pleasant outside the hottest hours.
Tuk-tuks are the alternative if you prefer not to cycle in the heat. A half-day hire to see the major monuments runs around 2,500-4,000 rupees; agree the price and route in advance. Within the modern town, tuk-tuks are easy to flag down for short hops between the bus station, train station, and the New Town hotel area.
Walking the entire ruins is theoretically possible but exhausting and the distances are deceptive. The Sacred City ticket is checked at several points around the perimeter; carry it with you all day.