Be prepared to pay a lot!
I havne't left bags at NRT for a long time, as the last time I did it (terminal 1), it was something like Yen1200 per bag, per day, and a new day starts at midnight. So if you get in at 10 pm, go into Tokyo and come back the next afternoon, each bag will cost you Yen2400..... 20 bucks or more. I'm not sure what the charge is now --- any one know?