Travel to the river: the journey to the river takes about 3 hours by bus from Kathmandu, 5 hours from Pokhara. Most groups put-in at Charaudi and take out at Mugling and others continue till Narayangarh to make it a 3 days trip. From Narayanghat you can visit the Royal Chitwan National Park.
On the river: the early sections contain class 2-3 rapids and build up steadily to class 4. 'Teen Devi' is a good class 4 above Benighat. Below here the river is class 3 as far as 'Upset' (4). After this the river winds its way down to Mugling at the Confluence of the Marsyandi.
Other options exist on the Trisuli for kayakers. A bus from Kathmandu to Betrawati or Trisuli bazaar can add another 15-20km of river to a trip. The advantages are that this section is away from the big road. It is predominantly lower in grade though at 2 - 3.