Actually River Kwai is pronounced and spelled in Thailand as River Kwae as Kwai is literally an insult meaning "a buffalo." This is equivalent to the "f" word in Thai society so be careful how you pronounce the river name:)
Now down to the lunching options:
1. You could be part of a group tour and have lunch on a floating raft while you are tugged upriver and downriver. Quite a nice experience if you do not feel seasick or riversick. It is also a very leisurely meal so do not sign up for this if your itinerary is hectic.