A section of Bunkyo Ward, Koraku is an entertainment playground. The centerpiece is the famous Tokyo Dome, but it is surrounded by an area called 'Tokyo Dome City'.An amusement park takes up a large part of this area. Rides seem to hang over the city streets, and they actually go through buildings and each other! They can all be ridden separately, or an 'all you can ride' pass can be purchased. The amusement park is built into the Tokyo Dome City, so it is free to walk around the rides.
Built halfway around the amusement park is 'La Qua', a building filled with over 70 shops and restaurants. There is also a natural hot springs spa built into the upper floors.Towering over everything is the 155 meter Tokyo Dome Hotel. I recommend having the buffet lunch at the 'Artist's Cafe' on the 43rd floor. Named after the artists that perform in the Tokyo Dome, make sure you reserve ahead of time to get a table right by the window. The city views are fantastic! If you are in the area at night, stop by the bar for the same great view, often with live music performances.
Also within the Tokyo Dome City is the 'Japanese Baseball Hall of Fame and Museum' (see next review). The Tokyo Dome itself hosts over 60 baseball games a year, as well as a number of other sporting events (boxing, American football, monster trucks,...). There are also special exhibitions, the most famous being the yearly 'International Great Quilt Festival' held every January (photo below).
The Tokyo Dome City also contains the largest JRA WINS horse race betting complex in Tokyo. Here you'll see many down-on-their-luck Japanese men hanging out, smoking heavily, trying to win their way out of disfortune. A large game center is beside WINS, in addition to games there are batting cages, bowling, and more.
For English information on everything I've mentioned, visit this site: http://www.tokyo-dome.co.jp/e/dome/
Access: Tokyo Dome City is located in the southwest corner of Bunkyo Ward. Korakuen Station (on the Marunouchi and Namboku Subway Lines) and Suidobashi Station (Mita Subway Line and JR Chuo Line) puts you right in the center of the 'City'. It is also a very short walk from Kasuga and Iidabashi Stations. Here is a nice map of the area:
http://gmap.jp/shop-11397.html
The park is not very large, but there are a lot of things for children to do. First, there are some rides including a small ferris wheel, merry-go-round, mini-train, and pedal-powered 'sky' cars.
Also in the park is an 'Animal Plaza'. In addition to some caged animals (monkeys, birds,...), there is a great petting zoo. Kids get a chance to get up close and personal with goats, deer, sheep, rabbits, and really cute guinea pigs! On a second visit to the park, the petting zoo was kept clean, but the waste in some of the other cages needed to be dealt with.
Many were enjoying a couple of small fishing ponds. This area was quite popular with older elementary school boys and their fathers. 
The park also contains some grassy picnic areas, some fast food outlets, and some great blow-up 'attractions'. There was also a live music performance the day I was there (traditional Japanese drumming, flutes,...).
If you just want to look around, the entrance fee is 200 yen for adults, 100 yen for children. For 1400 yen, and adult and child can enter together and they receive 12 tickets. Each ride 'costs' one ticket for a child, two tickets for an adult. Extra tickets can be purchased if necessary.