One weekend in Bangkok
If one of your days in Bangkok is a Saturday or Sunday, use this chapter of the itinerary. It is filled with attractions that are only available on weekends.
Bangkok is a shopper's paradise, and the Chatuchak Weekend Market in Phahonyothin has to be the most impressive market of the city. Getting there from Tha Chang is a pickle though, as Chatuchak can best be reached by Skytrain or Metro. Take a metered taxi to Ratchathevi Skytrain station (B100). A cheaper way is to walk to Tha Phan Fa, the closest Saen Saep Express Boat stop (near Golden Mount), and get to Ratchathevi from there. Then take the BTS north to Chatuchak Park (around B40).
Get up early and take the Chao Phraya Express Boat to Chang Pier. From here, you can take a canal tour through Thonburi, the western unspoilt side of Bangkok. Several companies arrange these tours from the pier. You have the option to go for one hour (around B800), one and a half hours (B1000) or two hours. All of them will visit Wat Arun and the Royal Barge National Museum and one of the floating markets. It is best to take this river cruise in the weekend, as then you will visit the Taling Chan Floating Market (as opposed to weekdays, when you will visit the unimpressive "Thonburi Floating Market".
Visit the Phaya Thai Palace.