There are several dive shops, for nearby divesites 1500 (plus 300 for equipment) Meticals, for further away, deeper reef divesites with a good chance to see Mantas (you might be asked for a advanced dive certifivat) 3300 (plus 300) Metical for 2 dives.
A quite nice daytrip, particular in a not so nice day but also for some shopping (exept off touristic things like wooden elefants, painted sunsets, sunglasses etc. there is not much to buy in Tofo).
Get there by (very full) Chapa for 18 Metical, about 1 hour or by taxi a bit faster and 400 Metical.
See the Sunrise! Tofo is the sort of place that is sometimes too easy to watch the sunset with a load of beers. Try the sunrise. The area is at its most peaceful and beautiful first thing in the morning.
Most stuff to do revolves around the water. You can dive or snorkel or see whale sharks. Theres a combined 2 hour trip for snorkelling and whale sharks for just $10! bargain!
You can also go quad biking for half a day or go kayaking in the lakes.