It is possible to take a one day boat trip out of Dhaka. We used a company that charged around 2,500 Taka per adult if there there at least 20 adults. The boat is spacious (and shaped like a peacock!) and we had it all to ourselves. Lunch and several touristy stops were included: we had a quick walk to an old estate, a stop for a swim (dolphins were also promised but alas), and a visit to a village where the laborers make “jamdani” saris. The “jamdani” clothe is a woven gauze and very expensive due to the labor involved in weaving it.
I enjoyed sitting on the rooftop deck, in a comfy chair, watching the river float by. Plus, there is a toilet on the boat. So the day out included two hours in traffic, six hours on the boat, and an hour back to Dhaka.