Well, I don't travel during the summer, but I'm going to venture a guess that it's going to be busy during the week right now because of summer vacation. Not as busy on the weekend, but I feel confident in saying take advantage of the FP system. If you had more time, I'd say hang out and read posts, but since you're short on time, I'll bestir my lazy hide to offer advice. Of course, the only thing I actually know about is traveling with a 5 yo princess.
Make sure that you have a flexible plan. It's going to be tiring for her, and nothing will make you more miserable than trying to haul her around if she's tired. Have an itinerary of some sort, even if it's just a list of the "must-do" rides. Without an itinerary, we found ourselves wandering from land to land in a very scattered way. Figure out whether she's going to want to stand in line for autographs or to meet characters. I like to plan at least one character meal during a trip, so that I can cheat--my kids get to meet characters while I laze about. Don't be afraid to plan some pool time--I was amazed how important a pool is to my kids. DO touch the golden apple at the entrance to Snow White's Scary Adventures, and listen. Look up at the window above the ride, and wait for a while if you don't see anything. (Someone's up there!) Make sure to do Fantasyland early, because the lines build up VERY quickly. ESPECIALLY Dumbo. Toontown is a great place for her to run around if you're tired and want to rest--let her explore Donald's boat, or jump in Goofy's bounce house. It's probably the last year that she'll be short enough to go in Goofy's bounce house, too. Toontown is a great place to get video, too--I love the video I got of my kids pretending to be electrocuted when they tried to open the Electric Company doors. Hey, she should be big enough to drive on Autopia. She'll probably be too short to go on Indiana Jones. She'll most likely be tall enough for Big Thunder Mountain Railroad.
In DCA, there's a water area, so if you go, have a contingency plan! My kids get soaked to the skin, and there's nothing worse than sloshing through the park in wet sneakers. Do It's Tough to Be a Bug before you let her go through the water, or she'll freeze to death. Definitely do the tortilla factory tour--she may not care about the whole thing, but parts will be interesting, and fresh, warm tortillas are YUMMY. She should be big enough to go on Grizzly River Rapids and Mulholland Madness, but not Orange Stinger. She'll be big enough for Tower of Terror, but definitely use parental discretion for that one!