To be honest, I have heard travellers say that they did not find there was much reason to visit Cienfuegos. I actually loved it, and it may well be my favourite city in the Cuba. Perhaps, the fact that most travellers dont find the city so interesting makes it even better, because it is quiet, not as touristy as the rest of the country. Cienfuegos is a UNESCO World Heritage site, a relaxed and breezy city, and compared to the rest of Cuba there are so few jineteros (locals who try to catch tourists for taxi drivers, casas particulares, and restaurants in exchange for a commission of the profits) here, that you will hardly feel harassed as in most Cuban cities.
Cienfuegos is right by the sea. It was founded in 1819. Like Havana, is also has a Malecon, a waterfront that eventually leads to Punta Gorda. One of the most interesting places to visit in Cienfuegos is the cemetery, with the tombs of the soldiers who died in the various independence wars are. Not too far from the city, the botanical gardens are a great place to spend a relaxing afternoon browsing to the various plants and bamboos.
Cienfuegos is a good starting point to visit the gorgeous Bahia de Cochinos (Bay of the Pigs) which is a perfect spot for diving and snorkelling, as well as El Nicho, a series of beautiful waterfalls of the Rio Hanabanilla with some lovely natural pools.