"Normal" Japanese teens.
 

I always wondered what the average Japanese teen does, besides go to school and all that. I assume not all of them go to Harajuku and dress up or go to Akiba and buy figurines and go to maid cafes. Are there any slang words widely used? The best english example I can give is when people say "dude" or "bro" and things like that. What do teens do in their off time after school, weekends, and holidays off from school like Golden Week and summer vacation?

The ones that do the things you mentioned are an extremely small minority. Remember, most Japanese teens live somewhere besides Tokyo.

In my experience, here are some common activities:

Study. Many of them go to "juku", which is a cram school they attend after regular school. These cram schools help them study for tests, especially entrance exams. Tests are everything in Japan, and literally 1 bad test can ruin your life. Schools in Japan are segregated according to how well students did on their entrance exams... if you do well, you'll get a place in an academic high school, and be taught the things you'll need to learn to go to university (which is pretty much a necessity for a salaryman in Japan). If you do poorly, you'll be sent to a technical high school or agricultural school, where they'll teach you manual labor such as auto mechanic or appliance repair or how to plant and harvest crops. You are not expected to ever be smart enough to enter a good university if you are sent to one of those schools, even though many of the kids there actually are very smart, they just did poorly on a test after leaving Jr High. I taught at both an Academic school and a Technical school while I was there.

Clubs. School clubs are extremely important in Japan. Almost every student belongs to 1 or more clubs, everything from flower arranging to kendo to various sports. In the US, clubs tended to be very small and only a few students belonged to any club at all. It's the opposite in Japan, only a few students are NOT in at least 1 club.

Work. I knew several of my students who had after school jobs, often helping their parents in their family business, such as a restaurant or store. "Mom and pop" stores and restaurants are still very common in Japan, more than the US has become, and teens are often expected to help out there too, after school. One of my students worked as a checker at the supermarket I usually went to, another at a restaurant, another at a convenience store, another at a small bookstore, etc.

Have fun. When they do get some spare time from the above 3 things, they will often read manga, watch anime, hang out at the mall, play video games, etc. But because of the above 3 things, they tend to have a lot less spare time for this kind of stuff than American teens.

Ah, that is sort of like our old Tripartite system, in the UK [They stopped that ages ago, though.] Now there are no entrance exams in the UK for normal schools. Everyone is put into groups according to ability inside the school itself since we changed to Comprehensive schools. It sucks that so many things rely on exams, and apparantly we are the most tested country in Europe which is fun too. I wonder how the UK and Japan compare in terms of exams >.< How many exams and levels are there in Japan?

Outside of the Anime/Manga/Video Games, there are also a lot of other types of fun activities. Japan is filled with entertainment, but a lot of them surrounds the various school clubs..

In the club veinn, I don't know about JET ALTS, but I was certainly encouraged to join a club myself since it is such a large part of the Japanese educational experience... I ended up joining the Tennis Club, and am kinda of sorta the assitant coach for the Boys team (even though I'm not very good) because the boys coach, Sato-sensei, is awesome. :)

I visited a few clubs when time permitted, and I helped out with the very short-lived English Club (not many interested at my academic school, too afraid to speak even when it was for fun, not for a grade). I played with the volleyball club the most.

It wasn't at my school, but the kids were incredibly shy, so it may have been that they were too nervous to do acting.