The Dusit District
The Dusit area is Bangkok's "new" royal city. It was established at the beginning of the 20th century by King Rama V (Chulalongkorn) upon his return from a European tour. Inspired by what he saw in the great capitals of the west, the district is laid out with wide boulevards lined with trees. The king established a new palace compound which he named Dusit, which means "Heavenly Garden." In the compound he first build a new palace, Vimanmek Mansion. After lying almost forgotten for nearly 80 years, the mansion has now been restored and turned into a museum dedicated to Rama V.
The present king still lives in the Dusit dristict, in the Chitlada Palace located at the eastern edge of the district. Note that the palace is not open to the public, and it is not even possible to see through the gardens and out-buildings to the palace buildings.
Between the old Dusit palace (Vimanmek Mansion) and the Chitlada Palace is the Dusit Zoo. The large park was originally the private botanical garden of King Rama V. It is still home to some distinctive tropical flora. The zoo is well kept by Asian standards, and very popular with Thais, not only for the animals but also for the cool green space and the great food. The Dusit Zoo is thought by many locals to have some of the best (and cheapest) food in Bangkok.
Actually, opposite the zoo, between it and the Vimanmek Mansion, are the current houses of parliament. Next to the government house is one entrance to the Vimanmek Mansion compound. Just inside of this entrance is the Royal Elephant Museum, and a little further in is the Abhisek Dusit Throne Hall.
Within a large fenced compound at the base of Ratchadamnoen Boulevard is the huge edifice of the Ananta Samakorn Throne Hall. Although for a time it housed the Thai parliament, the building has been restored for use as a throne hall and other state occasions. The plaza in front of the throne hall is where the annual trooping of the color is held, in view of the large equestrian statue of King Rama V.
Between the royal plaza containing the Ananta Samakorn Throne Hall and the Chitlada palace is one of the most beautiful temples in all of Bangkok, Wat Benjamabophit. Quite commonly known as "the Marble Temple" for its use of fine Italian marble, it's another one of King Chulalongkorn's contributions to Bangkok's architectural heritage.
Along the northern side of Krung Kasem Road from the river to Samsen Road is one of Bangkok's better-known garden centers. While you're unlikely to find anything you can take home, browsing the market does make the walk from Tewet Pier to Wat Indrawiharn or the Dusit Palace a bit more pleasant. If flowers are really your thing, then you might also want to take in the Museum of Floral Culture, which is also in the Dusit area.
The only hotel in the area of real note is The Siam. Opened in 2012, the hotel is very possibly the most POSH address in Bangkok, with private pool villas and other amenities.