The most famous building in Putrajaya, Perdana Putra is home to the Prime Minister's offices and is open for free public visitation. It is a mixture of Islamic and Malay architecture and is a strong example of modern Malay design, although a viewing from the outside is all you'll need since there aren't any tourist-related proceedings inside.
Between the Perdana Putra and the Shangri-La is Taman Putra Perdana, a lovely, quiet hillside park that gives some great views of the city. Taking them all in will give you a good half a day of cultural immersion while in Putrajaya.
Note:Putrajaya in itself is a well-planned, modern city, using a great mixture of cultivated greenery and attractive architecture to appeal to the aesthetics of travelers. Wikitravel has a good Putrajaya page.