Once you've done the deed and your resignation letter is given to the appropriate people, there are a few things you should bear in mind. You've indicated that you're packing in, for sure, and your responsibilities are just about over, but there are do's and don't's even at this late stage.
You may be asked to vacate your desk immediately after youe submit your letter: indeed, these days you may be urged briskly off the property by large and unfeeling gorillas. If you are asked to work out your resignation period, and you agree to do so, keep a low profile. Do not originate gossip or seek it out. If somebody tries to involve you, tell them that you don't think it's wise at this time for you to listen. Don't get involved in bitch sessions, although if they go on at coffee time with your usual group of friends you may find it difficult. Do the best job you can with any outstanding projects or with orientation of the person who'll be taking over from you. Don't shirk or slough things off, but don't let yourself be taken advantage of. An emergency four-day project on Thursday of your last week is something you can refuse! All this is in aid of leaving as good an impression as you can manage; you may well run into some of these people again!
(Incidentally, if you have to bring documentation up to date, concentrate on error situations, year-ends, and month-ends, in that order. It's doubtful that you'll have time to write everything up, but the day-to- day procedures will probably be familiar; it's the out-of-the-ordinary events that have to be looked after.)
Second is the exit interview. Not all companies have these, and those that do may not have a good reason for them. Ideally, the company is trying to learn from your departure if there are any systematic conditions that make employee turnover happen more often than it should, but it may also be trying to commit you to something or to make you feel guilty and blurt out something you'll regret. (H/R is a very difficult profession to master and most of the people you'll encounter are not very good at it). If you are asked to attend an exit interview, determine if you can refuse without consequences. If you have to attend, find out if you're going to be asked to sign anything; if so, get a copy and review it for nasty "gotcha's". At the meeting, you don't actually have to say anything more than you've put in your letter, but you can re-state it and add a detail or two that won't compromise you. You're in charge of your own statements here; after all, what can they do to you, fire you? But don't blow your cool and give them something on you.
Finally, you may be given a counter-offer to induce you to stay. There are limited circumstances in which you can accept this: such as delaying a retirement for a short while, or reversing a decision to resign because you couldn't get a necessary leave of absence. But if you have already lined up a new post, never, never accept a counter-offer. You will instantly ruin all credibility with your new employer - he'll never give you another chance; the word will get around, and you'll come off looking like a gold-digger rather than a good employee; your present employer will no longer trust you; and you may wonder why you had to go to the point of resignation to get what they now think you're worth. There are NO good consequences! You can perhaps imagine a counter-offer so enormous that you'd be nuts to turn it down - but there will always be a catch. Better to stay your chosen course.
It may also happen that your new situation turns out to be a disappointment; you might then be able to entertain an offer from your old employer, but you'll have to wait a decent period, typically six months or more, before you can ethically consider it.