If you are going to downtown Tokyo....
I have done that a couple of times only to get some electronic items that I cannot get back in NY. I went to Shinjuku Station - and outside there are some electronic stores.
It is a tough connection. The reason is that it takes an addition 30 minutes from the time you land to get to the terminal. It is such a LONG taxi time! So you have to figure in that.
Next, to get out of the airport, passing through customs, figure another 30 min. You should only travel by the train - which takes about 45 min. Buses take about 90 minutes. I forget but I believe both cost approx $30 US for each way, so it is a pricey trip outside.
Depending on what you are looking for in Tokyo, you need to decide what you are in the mood for. It takes time to find things around there!
By the time you return back to the airport, it is already 3 1/2 hours from the time of "landing" - if you spend up to 30 minutes browsing around or eating the excellent Sushi in Tokyo. Figure another 30 minutes for check-in and security.
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Going directly outside of the airport, there is really nothing to see. I've done that and spent a ton of money for taxiis to get me around for nothing.
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An alternative is to go through customs and stay inside the airport. The otherside of customs has a few stores and good food places (albeit a little pricey).
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Otherwise, after you land, spend an hour and a half walking through the small terminal. There are a few shops which basically sell the same stuff. A little walks away, (if you are interested in electronic stuff), there is a shop selling the latest camera, headphones, CD's etc. After all the walking around, Go upstairs to the AA lounge and relax. If you are a PLT or EXP member, you can even go to the Cathay lounge to the left of AA which has some more food choices and Wireless 802.11 access for free (you can get this free access within AA's lounge but keep to the left side for a better signal)
Even though you have a layover of 5 hours, the actual "real time" you have is around 3.5 hours due to the taxiing, check in, boarding time.