Most of Bangkok's sights are concentrated on the island of Rattanakosin, often referred to the "Old City". Out of Bangkok's hundreds of temples, the Grand Palace, Wat Pho and Wat Arun usually make up the top 3. The Grand Palace has an immense size, so expect to spend at least a full morning or afternoon there. Within the palace grounds is Wat Phra Kaew, the most sacred Buddhist temple of Thailand. Unlike other temples, it is not one building, nor are there living spaces for monks. Instead, it is a collection of highly decorated holy buildings and monuments. One of its buildings houses the Emerald Buddha, and while you might not expect it from its size, it is the most sacred Buddha image of Thailand.
Nearby is Wat Pho, home to the world's largest reclining Buddha image and a famed massage school. Take the ferry across the Chao Phraya River to Thonburi for the outstanding Wat Arun. The main structure is about 60 to 88 m high and it is surrounded by four smaller prangs. It is one of Thailand's most picturesque temples, and is engraved on the inner part of all ten baht coins. It is so recognizable that it even became the logo of the Tourist Authority of Thailand (TAT). If you climb it, and look closely, you will see that it is beautifully decorated with colourful broken Chinese porcelain pieces. Heading back to Rattanakosin, there are many other major temples you could visit, including the Golden Mount, Wat Suthat and Wat Ratchanaddaram.
Don't throw away the entry ticket of the Grand Palace, as it gives free entry to the Dusit Palace in Dusit. It is situated in a leafy, European-style area built by King Rama V to escape the heat of the Grand Palace. Its main structure is the Vimanmek Mansion, touted as the largest golden teakwood house in the world, but you could spend your whole day in the museums if you wish. There are many museums in Bangkok showing traditional Thai-style residences. Most visitors take a tour through Jim Thompson's House, the CIA-operative's mansion assembled by combining six traditional Thai-style houses, conveniently located near Siam Square. Ban Kamthieng in Sukhumvit, M.R. Kukrit's Heritage Home in Silom and the Suan Pakkad Palace in Phahonyothin are not quite as impressive, but still make for a nice experience. Rattanakosin's museums are mostly dedicated to history and culture, including the National Museum (about Thai history and archaeological remains), the Museum of Siam and the King Prajadhipok Museum. Bangkok has a small, but vocal art community, and you might want to visit the National Gallery or The Queen's Gallery, or one of its numerous smaller galleries spread over the city. Siam Square features the recently opened Bangkok Art and Culture Centre which has temporary art exhibitions throughout year.
Lumphini Park in Silom is the largest park in central Bangkok, and a good way to escape the fumes. Backpackers around Khao San Road can head for Santichaiprakarn Park, a small but fun park along the Chao Phraya River with a breezy atmosphere, usually with locals juggling or practicing tricks. It is built around the 18th-century Phra Sumen Fort with a nice view on the modern Rama VIII cable-stayed bridge. Zoos and animal farms are some of the more popular tourist attractions in Bangkok, but before visiting, please be aware that animal welfare in Thailand is not strictly regulated. Poor living conditions of the animals and inadequate veterinary care are examples of the sad mistreatment of the animal population. You can't go wrong at the Queen Saovabha Institute Snake Farm in Silom, as the staff takes good care of their snakes and they have a job of informing the public about the risks associated with them. Another nice family attraction is Siam Ocean World in Siam Square. It has a steep price tag, but at least you get to see the largest aquarium in Southeast Asia.