On the way home from Rotorua at the weekend we stopped at my favourite thermal area: Waimangu. If you're ever a tourist in Rotorua the best thermal / lake experience you can have is the round trip, which is a bus to Waimangu, a walk down the Waimangu valley to Lake Rotomahana followed by a boat trip, a walk across to Lake Tarawera, boat trip, and bus back to Rotorua. It's a really fantastic day trip.
This is one of the things you will see on your way down Waimangu Valley - the stream here is significantly hotter than the hot tap in your home. As is the lake it flows from! The photo really does not do proper justice to the colours.