Day trip to Bangkok - quick recommendations?

I'm heading back to Asia for a week or so and besides Kuala Lumpur and Singapore will also revisit Phuket and go to Bangkok for the first time.

Since I'm only going for a one day visit, what would people recommend?
I'm not much of a nightlife person, would prefer some daytime activities, that wouldn't necessarily limit me to museums or such. Good food of course is on the agenda.

My visit will be during the work week, but if anyone is free to show me around, by all means do respond.

cheers,
--Russ