Leaving the airport during a layover?

I will be stopping over for a 11 hour layover in the Narita airport, and will be making a quick trip to Narita to visit! However I've never left the airport during a layover before, and just want to know about the process so there are no surprises.

Could you give me a step-by-step on this? I've heard it takes up to two hours to go through immigration. Will I need to ask for a tourist visa or something? How about my luggage, will it automatically continue onwards to my final destination or will they assume that, since I've left the airport, the luggage is abandoned?

Any help appreciated, thank you!

(Also any tips on getting to/places to see in Narita is welcome as well :)

It depends on your nationality. If you come from a country which has a visa waiver agreement, the you can just enter Japan with your passport, but otherwise, it is probably more complicated. I don't know the details for that case.

I've heard it takes up to two hours to go through immigration.

These days, the lines in front of immigration are typically 0 to 30 minutes long. Rarely they can be longer. All in all it usually takes less than an hour from landing to having passed immigration and customs. Sometimes the entire process takes under 30 minutes. On busy days/hours, it can take over an hour. But two hours is extremely rare.

How about my luggage, will it automatically continue onwards to my final destination or will they assume that, since I've left the airport, the luggage is abandoned?

These things are complicated. You will be told when you check in. If not, you can ask when you check in. Or you can phone the airline company now, although a lot of phone operators don't have a fully accurate understanding of these matters, either. I have gotten wrong information before.

Also any tips on getting to/places to see in Narita is welcome as well

The following pages were created for people like you:
http://www.japan-guide.com/e/e6400.html

If your country of nationality is listed here http://www.mofa.go.jp/j_info/visit/visa/short/novisa.html
you will be eligible for entry without a visa.
You need to fill out a landing card as customs form, available on the 'plane.
Your luggage should be checked through to your final destination, so you don't need to pick it up and recheck it.
If you get your onward boarding pass printed at your starting point, you should be ok to renter the airside part of the airport, if not, go to flight connections and get one printed there.
Immigration queues are variable, depending on how many large aircraft with lots of foreigners on them have arrived at similar times - could take 5 minutes or 2 hours, but more likely it will be up 30 minutes.
Explain your plan to the immigration officer to explain your lack of address in Japan and show him your onward boarding pass. You could get a short transit visa if you are from a non visa waiver country. You then go through customs, which is usually only a minute or so.
Allow plenty of time to renter the airport on return, as there may be queues for security/immigration and the gates are often quite a walk. I doubt the airline would be informed that you have left the airport, but probably best to ask on original check in weather you need to tell flight connections in NRT your plans.

We tried it and simply were waved through with a photo taken. Narita is a wonderful town with a famous temple and market area. Everyone is very kind and will help you to order a meal or buy things without problems. We fell in love with Japan in that short stay and have been back for long visits 5 times since.

Thank you all! I'm Canadian, but I'll be entering in Narita coming from Jakarta. It doesn't sound like that'll make a difference!

Another quick question for you guys (sorry and thanks!), I want to see the city centre of Narita, walk down to the temple and see the park, come back up to the train stop (and have lunch along the way) - would I have enough time do you think to take a bus out to see Boso no Mura, then come back to the airport? Or would it be too tight?

This is the Bosonomura website it tells you how long it takes from JR Narita station: 20 mins by bus, 1-2 buses an hour + an 8 minute walk. So it really depends how long you're going to spend in Narita walking around, but you should have enough time (maybe do Bosonomura first then come back into Narita for lunch & sightseeing, that way you won't be 'clock watching' & worrying about waiting for a bus to get back to Narita at the end of the day.) http://www.chiba-muse.or.jp/MURA/e/index.html