Madu Ganga river is the best place in Sri Lanka for a boat trip, something like a river safari. The river is shallow and flows through the mangroves. After about 1 kilometer after the boatyard the river becomes very wide, there will be numerous islands. Each island is interesting in its own way and during the tour you will visit some of them. On the banks of the river there are huge monitor lizards and crocodiles. Children play just a few steps away from them :)

It's interesting that the children freely play near the river in 3 meters from the 2-meter long monitor lizards. ©Yuriy Buriak
A man on another boat gave us a monkey :)
The river banks are very scenic.
In the mangroves we were looking for crocodiles, but didn't find them.
Then we swam to the island, where a local family lives. The living conditions are really harsh, the house is built entirely of clay, no electricity, no amenities too. As I understand, they earn money on tourists. The man showed us various arts and crafts, we especially liked preparing of cinnamon.

The scourings will be used as a fertilizer and the bark will be used to produce spices. ©Yuriy Buriak
Cinnamon sticks are a curled bark, which is dried until is becomes brown.
We were shown how to weave a roof of palm branches and twist a very strong rope of coconut fiber.
We purchased cinnamon in sticks, powder and oil. Next, we moved to another island with a small temple and settlement.
We also stopped at a special place where fish is cultivated.
More photos of the Madu Ganga river
The boat for up to 8 people costs ~$50-60, the trip time is 3 hours. But in the comments people say that it's possible to find a boat even for $15 :)
Getting there: By train or bus along the coast to the Balapitiya station. The boat station is nearby, on the Galle Road, see the map below.