Most everyone who has heard of Tanna goes there planning to visit Yasur, the active volcano.
There are, however, a surprising number of other things to see and do on Tanna, including snorkelling, trekking, visiting waterfalls, riding horses, soaking in hot springs, visiting cultural villages and watching custom dancing, drinking kava, visiting coffee plantations, surfing, beaching, and attending festivals.
No matter what activity you are looking for, your accommodation host is the key to getting it done. The "tour companies" are generally part of the accommodation. Naturally you can expect a broader range of tours and activities from the larger bungalows and resorts than from the very small village bungalows which might not even have a vehicle of their own (or at least not one that works).
The more up-market accommodation - like White Sands Resort and Evergreen - have robust and roomy 4 wheel drive vehicles that can handle the rough roads of Tanna and still remain comfortable. The smaller facilities that do have vehicles tend to have pickup trucks with hard seats and cabins. These will get you there and back but may not be suitable for older visitors.
For some visitors the Volcano is the highlight of their visit to Tanna and it certainly should not be missed. But for others the people of Tanna are even more unforgettable.
There are several "custom" villages that you can visit. The villagers dress up for the occassion and do custom dances and show visitors how they light a fire, grow crops, and otherwise survive without modern technology. It's fun and also enlightening.
Some of the "custom villages" Like the Black Magic Tour, have a separate area set aside from the main village itself. The people seem to enjoy the visitors more than those custom villages where the people have tourists right where they live on a day to day basis.
Tanna produces Vanuatu's finest coffee so be sure to visit a coffee plantation and bring some Tanna Coffee home with you.
There are no SCUBA refill facilities on Tanna so this limits underwater excursions to snorkelling. The reefs are nice and there is an underwater cave on the northern end of Tanna that opens into an air-filled chamber.
You can also watch whales from the shore and see huge sea turtles crawling up on the beach - if you are there during the right season.
There are not many places where you can surf but there are some on the southeastern corner of Tanna.
The vast majority of the people of Tanna walk or ride horses from one place to another. Fuel costs more than $4.50 a litre and there are very few vehicles. This means that there are walking and horse riding trails everywhere on Tanna and if you are an avid treckker or horse rider you can arrange for some really exciting walking adventures between the mountain villages.
Check out all the tours and activities on the Rocket Guide to Vanuatu. This interactive program gives you a complete understanding of every aspect of Vanuatu tourism (It was designed for educating travel agents). You can download a copy from their site or order the program on DVD - it works on both Windows and Mac.