This is something I would really like to recommend!

A big reastaurant ferry, "Chao Phraya Princess", with indoor as well as outdoor (recommended!!) tables is leaving from the River City pier about 7.00 PM, and you will have the most beautiful and romantic trip up the Chao Phraya River enjoying a set-menu, live soft music (which allows you to talk), and the view of the Bangkok skyline and not least the temples which are all litten up.

Take a look at this website:

Bangkok Airport Transfer and Bangkok Tour

The smaller traditional style cruises are all having a roof which doesn't allow you to enjoy the sights, but if you prefer that you may chech this site:

::: ORIENTAL ESCAPE ::: Thailand travel agency offers high quality tour & sightseeing, promotion package, transfer, hotel reservation service in Bangkok Chiang Mai Phuket Pattaya Hua Hin & Other destination in Siam Land of Smile

But do go for it!