Wat Arun is a little unusual as it's really the only major tourist attraction on the Thonburi side of the river. It is open every day from 8.30am to 5.30pm. Officially, entrance to the Wat is 20B and the ferry crossing is 2B. Sometimes if you walk around the Wat grounds, you will be requested for a donation (with a book showing how everyone else gave 100B/200B), and sometimes the staff at the pier will cite some excuse and charge another 10B or so to let you through.
Other attractions fairly easy to reach from here are Wat Pho, Wat Phra Kaew, the National Museum, Chinatown the Ko Rattanakosin area, and the Banglamphu district.
Due to its location, by far the best and most common way to get to Wat Arun is on the river. The Tha Tien express boat pier, at the southwest corner of the Grand Palace / Wat Phra Kaew, is opposite Wat Arun and boats leave every few minutes. You can get to Tha Tien on the Chao Phraya River Express boats from any other pier, or take a taxi to it. Buses that go near Tha Tien are ordinary buses 1, 25, 44, 47, 62 and 91 which stop on Maharat road. There are also plenty of Thonburi canal tours that go here.
Wat Arun Thailand