There are four highly recommended Inca ruins outside of Cusco [1], which many Cusco agencies offer as part of a rushed, half-day tour. The ruins all accept the boleto turístico or they charge US$5 per ruin and are open 7 a.m.–6 p.m.; after-hours visits are possible at Sacsayhuamán [2], the ruins closest to Cusco.
Peter Frost’s excellent guide to Cusco, Exploring Cusco, details some interesting walks around this spectacular landscape between the four sites, which are littered with aqueducts, Inca roads, caves, shrines, and carvings.
All the ruins lie close to the road that runs between Cusco [1] and Pisac [3]. An enjoyable way to see the ruins is to take a Pisac bus or taxi to the farthest ruins, Tambomachay [4], and walk the eight kilometers back to Cusco, visiting all the ruins along the way (this walk can be shortened considerably by just walking between Q’enqo [5] and Sacsayhuamán [2], a distance of one kilometer).
Occasional robberies have been reported in this area, so it is better to walk in a group of two or more during the early part of the day.
Additionally, there are very interesting ruins along the road that heads south to Lake Titicaca [6]. These cultural sites, from pre-Inca, Inca, and colonial times, form an interesting day tour from Cusco that an increasing number of agencies are offering. These landmarks can also be visited on the highly recommended special tourist buses between Cusco [1] and Puno [7], which hit all the sights described, head over La Raya Pass, and then keep going to Puno and Lake Titicaca.
Links:
[1] http://www.moon.com/destinations/peru/cusco
[2] http://www.moon.com/destinations/peru/cusco/sights/outside-cusco/sacsayhuaman
[3] http://www.moon.com/destinations/peru/the-sacred-valley/pisac
[4] http://www.moon.com/destinations/peru/cusco/sights/outside-cusco/tambomachay
[5] http://www.moon.com/destinations/peru/cusco/sights/outside-cusco/q-enqo
[6] http://www.moon.com/destinations/peru/lake-titicaca-and-canyon-country/lake-titicaca
[7] http://www.moon.com/destinations/peru/lake-titicaca-and-canyon-country/puno