The Mount Somers Track is close to Christchurch, offers great views with impressive rock formations, and can be enjoyed as a LONG and hilly 26km day hike or spread out over 2 or more days if you choose to stay in Pinnacles and/or Woolshed Creek Hut.
The Mount Somers circuit is a tough 26km adventure, but it rewards you with constantly changing but beautiful scenery. From the start at the Sharplin Falls carpark, the trail immediately climbs upward through lush forest. As soon you reach a highpoint, you descend again to cross the river (via a swingbridge!) only to start another upward climb. You eventually break the tree line and start to see some of the amazing rock features and subalpine terrain. You stay above the trees for quite a while, but the up-down-up-down continues throughout the circuit.
Unless you're feeling super fit and up for a challenge, plan to do the loop as an overnight trip rather than a long day trip. However, that being said, my friend and I did the whole circuit as a day-trip (from Dunedin!) in mid-June when the days are short, so it's worth considering the day-trip option, especially during the summer and if you're setting out from Christchurch.
If you choose to make it an overnight trip, the first hut along the circuit (if you do the loop counter-clockwise) is Pinnacles Hut, and is only 5.2km from the carpark. The next hut you reach is Woolshed Creek Hut, which is about 11.5km from the start, so it makes a better mid-way destination. The circuit track doesn't take you to the summit of Mount Somers, but if you're keen to do the out-and-back summit detour you'll have the chance about 3/4 of the way along the circuit. We didn't have time to explore the summit, but I reckon it's a worthwhile excursion.
More track and hut info can be found here.