Sukhumvit is about 10-15 mins on skytrain to river depending on connection. (Saphan Taksin station..quite a long staircase to river level).

Bangkok will be very hot during the day, and hot in the evening. I always wear long trousers in the city, feel less like a tourist and its more comfortable when visiting areas that are well air conditioned, or a temple.

One evening diner cruise worth considering are the Manohra river cruises on converted teak rice barges... will pick up and drop off from the Saphan Taksin pier that links with the sky train...alternativly take the Bangkok Marriott Resort & Spa boat to the Marriott, the rice barge evening dinner cruises are based from there. Its a nice hotel to while away an hour before your cruise starts.