No matter what time we've arrived, we've always been lucky with queues in that there haven't ever been any, but I know others here have reported long lines (and we've dropped off friends who've been flying home sooner than us and witnessed them), so I think it's just the luck of the draw.
Like Netty, we tend to spend some time looking around Downtown Disney once we've checked in. If you're planning on doing that, there's no point turning up much before 9.30am because the shops don't open until 10am. Lately, we've been using online check-in and, whilst there's nothing to stop you doing both that and Downtown Disney check-in if you want to get rid of your bags, we don't bother.
On several occasions, we've been to a park on our last day, but no matter what you do, you can't prevent that "going home" feeling and, even with a later flight (ours is usually 7,30ish) you can't risk leaving it too late to head to the airport. On a couple of occasions recently, we've booked an extra night at our villa accommodation so that we can go out shopping or maybe to Downtown Disney, have some lunch and then go back to the villa for a swim before showering and heading for the airport.
Orlando International Airport is actually a very pleasant place to spend a few hours. Our tradition is to head up to the Hyatt where we either sit in the lobby lounge with a drink, or grab a bite to eat in McCoys (the casual restaurant just off the lobby). There are some very nice eateries in the airport itself, but we enjoy the calm of the Hyatt.