Most travellers do Polonnaruwa as one stop on a Cultural Triangle circuit, but if you base yourself here a couple of nights there are good day trips. Sigiriya, with its lion-rock fortress and frescoes, is 90 minutes' drive west; Dambulla and its painted cave temples are another half-hour beyond. Anuradhapura, the older capital, is two hours northwest.
Closer to home, Minneriya National Park is half an hour south-west — its tank attracts large herds of elephants in the dry months from July to October ("The Gathering" is one of Sri Lanka's most reliable wildlife spectacles). The much larger and quieter Kaudulla and Wasgamuwa parks are also within day-trip distance.