Hunt down a bargain or two, purchase a cute souvenir or sample some regional fare at Khlong Bang Luang Floating Market. This busy place will give you a real feel for the local community and culture.
You'll find this community marketplace in the urban neighborhood of Wat Tha Phra. If you're on a budget, you'll be happy to hear that it's free. Throw in nearby sights like Grand Palace, Wat Pho and Wat Arun and you have yourself an itinerary for a fun day out.