You've never heard of history extension? Oh noes!
I'm doing it. It's alright.
Make sure you're passionate about history. And make sure you're passionate about writing essays. I've found it to be a lot of work for a one unit course (to be honest, I've done more work in extension than I have in modern), which is also a slight drawback. If you're interested in doing a major work (which is the big drawcard of the course), IMO you should consider EE2. Glitterfairy won't have much difficulty convincing you it's the best thing since sliced bread.
The course is set out in three parts (well, this is the way I see it).
'What is History?' - 1st Module
You look at the philosophy behind history. You'll read a stack of sources at the start of the year. I really loved this section. Got into some meaty discussion on the nature of history, truth, objectivity, etc. etc. It's downside is remembering quotes! Which i'm not always brilliant at.
'Case Study' - 2nd Module
There's a stack to pick from, and it's just like your national case study in modern, except more focused on historical debate than events. I did "Jesus", many do "JFK" or "Elizabeth I". I'm not so keen on this one, probably because it's a lot of the same kinds of material, and there's not as much room for debating your own ideas. (But many disagree!)
And of course, your major work...
Which is composed of a few different parts, and can be a lot of fun. The word limit may kill you though. Mine was 2000, which was awful.