There are loads of National Parks for those wanting to watch Tanzania's wildlife. You can gain entry for around $100 US and benefit from a tour and perhaps a night's accommodation. The better parks, though packed with tourists, are found in the north of the country. Ruaha National Park is the best in the south locals actually say this is the best park, especially if you want to see wild animals as opposed to semi-tame ones in the northern parks. Don't just be sucked into the tourist circuit in the north; the south offers great parks and towns base yourself out of Iringa, and you will feel less of a tourist and more of a guest if you travel this way.
There is a wide assortment of volunteer organisations sending volunteers and interns to Tanzania to do work in health care, orphanages, education, and development projects. Finding a paying job may be more of a daunting task, taking more time and making use of local connections, but a job could be certainly obtainable when sought hard enough.