I spent a few times here while on my trip, and its not just any mall, there are a lot of other things to do besides shopping!
Be sure to check out the Aquarium at the top level, its excellent, has many kinds of fish and other sea creatures from around Japan, including the Japanease Crab its a beast! The penguins as well were also a plus!
The planetarium wasn't open the times I tried to get in, I almost went back to the aquarium again but I was on limited time!
'Sunshine 60' observation deck.
This is the highest building in Tokyo, and a good place to get an overview of the city. The elevator ride is fun too... the guide turns on black lights to highlight the tacky day-glo, pseudo-astronomical artwork on the elevator walls. Try not to giggle. Also, there are all sorts of amenities at the top: an arcade, several gift shops, and a café. On the day I visited, they were conducting an origami class.
If you have time, consider wandering through the peaceful neighborhoods just east of here. Tokyo's last remaining tram line runs past here, and is a lot more interesting than the subway or the Yamanote Line.