There are many buses to Butwal and to Sunauli/Bhairawa. There is one government Sajha Yatayat bus daily to Kathmandu There is one bus to Pokhara and others coming from Butwal. There is a bus to Kakarbhitta. To go to other place in the Terai, you change buses at Butwal.
You can also take one of the more comfortable and faster semi-tourist buses that go between Pokhara and Sonauli. These buses run along the Siddhartha Highway, so you have to get off at Bartun, which is the village three km from Tansen at the road that turns off to Tansen. From there you can either get a bus to Tansen or you can walk up the steep 1km short-cut that begins north of the intersection.
The highway running north–south has some great views, so you may want to ride on the roof. If you are coming here from Pokhara for the best views sit on the right side of the bus.
There are no rickshaws or taxis in Tansen. The better hotels can usually arrange a vehicle or a motorcycle hire. You can rent a bicycle at some guest houses.
Srinagar Danda is a 1600m high hill just north of Tansen, from where there are excellent views over the Kali Gandaki River of the Himalaya. You can see Kanjiroba in the west to Annapurna and to Langtang in the east.