Mantua, recognized on UNESCO World Heritage list as an example of Renaissance town planning, is very much worth an intraday visit. Three to four hours is likely enough to get a good impression of the town and what it has to offer.
A couple of major squares, Piazza delle Erbe ♥ and Piazza Sordello ♥♥, host a number of interesting architectural monuments. The former is more commercial. The latter is significantly larger and formal-looking and is home to some of the main points of interest in town. Duomo ♥ is traditionally decorated and impresses with the unique details of every chapel. Ducal Palace ♥♥ is worth a visit as well, with two dozen increasingly impressive spaces comprising the tour. There is not much furniture but magnificent ceilings and wall frescoes and decorations. Audio-guide is hardly necessary, all rooms are signposted in English. The same entry ticket also allows access to nearby Castello di San Giorgio ♥ with one splendid staircase and a room full of Andrea Mantegna’s frescoes (Camera degli Sposi) open for viewing.
Other significant attractions in town that we did not find time to visit are Palazzo Te and Basilica di Sant’Andrea.
The town of Sabbionetta is linked with Mantua on the same World Heritage entry as another example of Renaissance town planning. Our less than hour long stop there failed to impress us. We did not find much worth of a look. Planned grid of streets with some nice buildings comprise the town core. Pleasantly-looking main square houses not exactly exceptionally looking Ducal Palace and a church. All potential points of interest, including churches, were closed on a Saturday, so we could not form any opinions of their interiors.