The fireflies aren't guaranteed (one cruise operator claims he's offered them cash for the electricity costs and umbrellas for the rainy season to get them to show) but they are magical if you're in luck. The further north you go the more and more rural the scenery becomes, so it's worth taking a trip of two or more hours.

The Boatman comes highly recommended. Bart from Belgium offers private cruises and often gets to places that other operators don't, charging between $40 and $70 -- depending on the number of people -- for three-hour trips along parts of the river that he has well explored. 

For group tours and those on a tighter budget, Naga House and Captain Chim run great value sunset cruises for $5 a piece, including one free drink.