Just below the surface in the basements of department stores are food wonderlands known as depachika (デパ地下 ~ literally department store underground). Many of these are so large that they're considered food theme parks of sorts. They often consume 1 or 2 entire floors. Depachika are a great place to pick up bento, snacks, groceries, gifts, souvenirs and desserts.
BentoDepachika are a bento wonderland. They might have 100s of prepackaged varieties.
Most also have build your own bento options.
Many urban Japanese are too busy to cook and practically live off bento. Department store bento are usually higher quality (and more expensive) than what you'd find at a convenience store.
Everything has a reasonable price to quality ratio. For example, you can sample some reasonably high quality sushi for a fraction of the cost of a restaurant.
Some department stores feature public areas on the roof where you can eat your bento. These range from elaborate Japanese gardens to drab concrete seating areas.
GroceriesIf you have the money, do all your grocery shopping at a department store. The quality and selection usually beats any supermarket out there. They're a little pricey. You'll notice many well-off people shopping in the basements of Ginza.
GiftsIn Japan it's customary to give Ochugen (mid-summer gifts) and Oseibo (year-end gifts) to practically everyone you know. This may include customers, neighbors and family. It's especially important to give gifts to anyone who has done you a recent favor.
Ochugen and Oseibo gifts are usually practical things. You can even give laundry detergent gift baskets. Food is the most popular choice. Gift sets of beer, sake, desserts or expensive gourmet foods are available at depachika. Often you buy these gifts and have them delivered. Depachika may display plastic models of the gift sets rather than the real thing.
DessertsDepachika are amongst the best places in the world to buy desserts. Some have 50 or more vendors selling fine French and Japanese desserts. It's a dizzying experience for lovers of fine cakes, pastries and chocolates.
It's also a great place to find those famous 15,000 yen Japanese fruits (e.g. cantaloupe).
Many Japanese desserts, candies and chocolates are portable – they make excellent souvenirs.
RestaurantsThere are a few tiny restaurants in many depachika. A good place to eat decent food when you're alone (max 2 or 3 people).
Samples and EventsDepachika often have some kind of event or promotion going on. There are usually some interesting food samples to be had.
On Japanese mega holidays such as Golden Week depachika all over Japan hold travel themed food events (e.g. Hawaiian food). The idea is that those who couldn't travel can still enjoy some exotic food.