One of my favorite times in the parks is actually after closing.... no rushing to get to a ride... you can just relax and stroll around enjoying the atmosphere... Last trip we saw the MK opening ceremony and the castle "Kiss goodnight" on the same day without leaving the park.. LOL
This, right here, without question is the best tip you can get, I believe. Late night -- I mean, Extra MH from 12-3 or so, the parks are truely deserted, literally only hundreds of people are acattered throughout the Magic Kingdom, so it's all yours.
The Kiss Goodnight is awesome, but the time before that is cool too.
I have a link from a trip a few years ago of us at EMH in the MK. It's not a long video but it shows how empty the parks are late night. It's about 30 seconds and it's mainly about a CM named Carl doing water art on Main Street. Still, in the background, you can see just how deserted the park is. I see now, after having watched the video again for the first time in a while, that I got pretty excited about his water art, but it was late, late and I was on a DW high. And if you can't be goofy on Main Street in DW, where can you be?
http://www.youtube.com/watch?v=afQszv83s5k&feature=related
If you can do rope drop and then make EMH until 3 a.m., you are -- in my mind -- hard core Disney and will reap the benefits of a line-free park for a couple hours. It's an amazing thing to do, run from ride to ride without waiting.
Small World, step right up. Peter Pan, can do right now! Buzz Lightyear? Why not? Haunted Mansion? Twice? Don't get out of the Doom Buggy? Done. BTM? Huh? Is that a line of four people? Better do FastPass. You get the idea.
THAT is my Number One Tip.