The Most Awesomest Store in Japan! Don Quixote Akihabara
Mega Donki stores contain many of the same goods you see at regular Donki stores, but they are toned down as far as the really goofy goods go (a very minimal costume/party section for example). On the other hand, they have a bigger food/candy/liquor selection, serving as a grocery store. And they have a larger selection of other goods you might find at K-Mart, Target, or WalMart in America (home, office, clothes, toys, etc).
There are Donki (Don Quixote) locations all over Tokyo and Japan, and Mega Donki stores in larger suburbs outside major cities. Their English website lists their stores and provides a map, too! Every Donki will have a good selection of everything you see here, but I think the Akihabara store is the largest (non-Mega) location and certainly has the best range of goods for browsing!