Tsu is Mie's capital, a low-key city on Ise Bay between Nagoya and Ise. It does not have the pull of Ise Shrine or the food fame of nearby Matsusaka, but it has enough history and museums to make a useful, pleasant stop if you are already moving through the prefecture.
The old centre is around Tsu Castle, where moats and stonework survive from the castle town. Nearby, Tsu Kannon keeps a more everyday temple atmosphere, while MieMu gives useful context for the wider prefecture.
Tsu is best treated as a half-day city. Come for the castle park, a museum, and a shrine visit, then continue toward Ise, Matsusaka, or the Kii Peninsula.