In the interest of providing an opposing viewpoint, here are three areas of Yokohama that I think are worth leaving Tokyo to check out:

Minato Mirai: Like Odaiba, but nicer.

Motomachi/Chukagai: Fun place if you like old buildings, chic shopping, and Chinatowns.

Osanbashi/Naka-ku: OK, the pier itself isn't so interesting, but there are a lot of parks and neat old brick buildings in the area that you really can't find in Tokyo. The Red Brick Warehouse, an actual warehouse from the 19th century that was reconverted into a shopping center, is also a unique experience.

And if you want to check out a baseball game, you'll probably find much better seats available in Yokohama Stadium than in the Tokyo Dome.