"Day trips" from Kananga is something of a relative term given the infrastructure. Nothing here is easy, and what would be a short excursion elsewhere may take most of a day just to reach.
Surrounding villages are the most accessible option. A moto-taxi can get you to villages within 20–30km of the city, where Luba and Lulua communities continue traditional ways of life. There are no organised tours; the best approach is to hire a moto driver who knows the area and is willing to guide you. Ask at your guesthouse.
Kasai river areas offer fishing communities and river life that is quite different from the urban experience of Kananga. The Kasai and its tributaries support a significant population, and the riverbanks are active with boats, nets, and the commerce of a river economy.
Mbuji-Mayi is the DRC's diamond mining capital and the largest city in the Kasai region, roughly 200km east of Kananga. It is a significant place in its own right, but road access is difficult and slow. If you are planning to visit both cities, fly between them rather than attempting the road.