A visit to Polonnaruwa takes time and energy. We are talking about a small city here and there a large number of individual areas to see. To 'do' the lot would take days and, unless you are an archaeologist, you would almost certainly be 'ruined out' by the end of the first day.
Most people therefore go for the main attractions like The Quadrangle, Shiva Devale, Gal Vihara, Vatadage,the Royal Palace Group, Island Garden and Lankatilaka. Even to do these, you will need transport and at the very least 5-6 hours which, in the temperatures (30-33C) existing for most of the year, is seriously debilitating.
However, it is worth the effort as the ruins are all impressive, though they do require a certain degree of imagination to conceive of the sheer magnitude of the whole site and the manpower that must have been required to build it.
Polonnaruwa is a must for any visitor to Sri Lanka and fully repays the exertion expended.