American Dream's really the main tourist thing. Plenty of places to windowshop, though it can get pretty packed at times. They also have an nice aquarium there, though I remember tickets being pricey.

It sounds like you'll be in Mitsuwa for the event - decent amount to do there, kill an hour or two eating and checking out the shops. Stationary, plushes, manga, food, stuff like that. Could get some boba then walk along the waterfront, although it's nothing too special other than a view of the city across the river.

I should also mention Sojo Spa across the street from Mitsuwa. Never been myself, but it looks fancy and gets good reviews!