If you want a good hard workout go in at the end of grey road up the Mahaukura track to Pirongia and back down the Tirohanga track. Its posted as an 11 hour walk but you can do it in a long day however that would mean missing out on a night in Pahautea hut. Theres something about that place. You could do a quick day up the Tahaunui track. A quick easy climb with some views at the top out towards raglan.
If you are into SAS type training I can recommend the bell track on a really wet day. A 3 to 4 hour warm up to start and then a 3 to 4 hour climb and mud wallow up to your waste onto the cone and then to pahautea hut. Ive done it but not willingly again.