Otago is the heartland of New Zealand. Dunedin is the capital of this province and has been discussed in another tread in greater detail, so that this can focus on the wilderness of the plains.
Otago is a fantastic part of New Zealand, and my words here will not be able to do it justice. The golden plains with their amazing rock formations from standing pinacles, gorges, castles, among a few, are amazing. The area around Alexandra has some of the best rock sites in New Zealand. And contrasting the rocks are the golden plains which are amazing no matter what time of the year. But obviously in winter they turn a lovely white, and some of the best ski slopes in New Zealand are found here.
The Otago Rail trail is something I have yet to do by bike (which is the best way to do it) however you can start the wonder of this journey using the old rail system which is still running out of Dunedin Railway. This old rail line has some amazing engineering feats getting across the gorges that are hard to believe they are standing after so long and looking so perfect.
There are two key towns in the area that must visited: Cromwell and Alexandra. Both of these towns have important historical aspects and are also useful to use as a base to explore the surrounding nature. Alexandra has its famous mountainside clock and a very cute museum that are must dos. Cromwell, used to look very differently until the creation of the dam (and lake) which actually flooded a fair section of the town. These days there are some remanents of the old town left by the riverside which are great fun to meander through.
Nearby to Cromwell is New Zealand desert. Seriously. Bannockburn, however, isn't well known for that fact, but its incredible wine. Some of the best red wine comes from these parched lands and well rambling around there its a good way to soak up some moisture trying free samples. Like Cromwell, Bannockburn has a history of gold mining and this is evident in the sheiling that is dotted around the place. Although the best evidence of this is just up from Cromwell on the side of the road. Called Fishbones.
Wanaka is another great place in Otago that must be visited. The views onto the Mt Aspiring National Park from Lake Wanaka are amazing. However, it is slightly expensive to stay in Wanaka, and camping in Mt Aspiring National Park is a better option or just further up in Lake Hawea. The Aspiring Camp Site though isn't too bad and it has a sauna.
Mt Aspiring National Park which crosses between Otago and Westland has dramactic colour contrasts (very photogenic), and multiple walks that offer a wide variety of environments to explore. Our favourite is the Rob Roy Glacier walk. Takes a wee while to get there, but the views on the glacier and of the streams are amazing. Not to mention the kakapo and other wildlife are great fun. Do make sure you prepare as, as with anywhere the weather changes fast. We had sun, rain, wind, hail, and it started snowing just a bit further up from the glacier (although we were heading out by then).