Rotorua is sited right next to a lake that bears its name, but that is not the only lake in the region.
Rotoiti lies to the east, and you can take a really nice drive along the southern shore of the lake by following State Highway 30 (head out past Rotorua Airport and take the Whakatane turn-off). At the end of Rotoiti there is a really great bush walk at Hongi's Track. Go down Tamatea St to find the track entrance. Past Rotoiti to the east are Rotoehu and Rotoma, the latter being a great place for a swim.
Off State Highway 30 at Ruato Bay you can find the amazing drive through the forest to Okataina.
To the south of Rotorua, take Tarawera Road to swim at Okareka or Tikitapu (Blue Lake), gaze at the privately owned Rotokakahi, or swim at Lake Tarawera.