First, it is a starting/ending station of the Tokyu Setagaya Line, one of only two remaining streetcars in Tokyo.
Second, is the Carrot Tower. It is Setagaya Ward's tallest building, and as it was built to attract people to Sangenjaya and the ward itself, it has a free observatory on the 26th floor.There are many observatories in Tokyo, but I like this one because Sangenjaya lies outside of the Yamanote Line and their are no other tall buildings nearby. It gives a very unique perspective of the city.
The best way to enjoy the observatory is to have lunch there. Since the Carrot Tower is owned by the Setagaya Government, the restaurant has a cafeteria feeling, but the view and tasty food more than make up for its lack of style. The menu is all in Japanese, but if you can read katakana, you can read most of the choices. Best of all, lunches start at only 1100 yen and usually include salad and coffee/tea.The Setagaya Public Theatre is also located in the Carrot Tower. And, I am happy to say, they actually have an English website: http://setagaya-pt.jp/en/
The third reason to visit Sangenjaya is because it is home to one of Tokyo's two Samba Festivals (the other is in Asakusa). It is called the 'Sancha Matsuri' and in 2008 was held on August 23 and 24. The festival commemorates the historic relationship between Japan and Brazil, and it features an impressive Samba parade, live performances, and booths selling a wide variety of products.