So, you've given your notice at your job. Now what? First off, stop coding. That's right. Finish whatever small task you're currently working on, but for the love of god, stop writing more bugs. Of course, you want to get your manager's sign-off on this, but in my experience managers generally accept that pumping out just one more feature before you leave isn't the best use of your time.
So, what can you do to be useful?
Do you remember what documentation is? It's that thing that nobody ever produces. You know, the stuff you wish you had when you started this job, and inherited a mess of legacy code? (Every developer thinks that code they didn't write sucks. The next guy is going to think you're code sucks. He's probably going to lobby to re-write the whole damn thing.) Wouldn't it be nice if he had a road map to work with?
Never having written any documentation, some developers find themselves at a loss about WHAT exactly they should be documenting. Maybe this will give you a starting point.