Waimangu Valley is a really incredible place. It involves about a 2-3 hour walk down a valley towards the lake. Along the way you pass some amazing natural landscape such as the Inferno Crater (a mind-blowing blue hot spring). There are 3 conveniently placed bus stops along the walking track. Be sure to wear good walking shoes as parts of the track are steep. Additionally note that the walk up Mt Haszard has a steep incline. But you are rewarded with an amazing view of the valley at the top.
Here are the few that must receommanded activities for your kiwi experience of lifetime.