Buses to Dar es Salaam (at least 9 hours) depart early in the morning. Get a seat near the front as part of the road is unpaved.
For transport heading further inland ask around at the bus stand.
Towards Tunduru there is a modern road as far as the village of Mangaka but the next section is not yet built. If you want to see the Unity Bridge which goes over the Ruvuma River to Mozambique there is a well made murram (or laterite dirt) road towards the bridge from Mangaka.
There is reported to be a rock art site within walking distance of Masasi.
There are a few small hills around Masasi and it is possible to hike up them.
There are no tourist guest houses, but the town is a major centre in the cashew nut trade so there are a lot of guest houses in the Tsh 4,000-30,000 range. You will get what you pay for.