At the end of its mine tours, you can try some of the work yourself. This company also offers tinku excursions.
South American Tours
Visits the mine San Miguel la Poderosa, in addition to offering the standard tours.
Not really designed for walk-ins, these are mostly private tours that are pricier but of a higher quality. Worth booking in advance.
Lonely Planet review
Runs some of the best mine tours, with ex-miners as guides (Efrain Mamani is highly recommended); 15% of the profits go to the mining community. They also offer guided mountain biking to Tarapaya (B$150), tinku excursions (a four-day trip costs B$800) and a nine-day trek along the ancient llama caravan route to Cochabamba.