The Sacsayhuaman Ruins is a UNESCO World Heritage Site, close to Cusco; you can get there from the city center via a series of stone steps, which shouldn’t take more than 45 minutes, but public transport is also available.
Sacsayhuaman is a pre-Inca structure. It was built by the Killke people, but later the Incas rebuilt the site; therefore, it is considered one of the best examples of the construction skills of the Inca civilization.
If you are looking for a recommendation for a half-day excursion, there is probably nothing better than a leisurely walk from the historic center of Cusco to the Sacsayhuaman Ruins.
No matter how many ruins you have already seen in Peru, this one is simply a must see. Not only will you have the opportunity to admire what was left, but the views from this place are impressive.
If you have more time, it is possible to take a bus from Cusco and explore more nearby ruins, such as Qenqo, Puca Pucara and Tambomachay.