I always go in June/July because I am a teacher and that is my longest break for a vacation. I love going in the summer because of the water parks and all of the parks are open later! We do usually go for at least 2 weeks though.
If you really want to go in the summer, it will be crowded, but manageable. The lines do get long, but usually it's the rollercoaster type rides with the lines that are 1-2 hours long.
Last year from July 12-28th, we never waited in any lines longer than 20-30 minutes!
We go first thing in the morning and use EMH at the MK then leave at about 11:30-12:00. We also use FP on all rides if at all possible. We take a break in the middle of the day when it is the hottest and go back to the resort to swim, play games in the arcade, or watch TV. Then, around 3:00 or 4:00 we go back to a non-EMH park (usually a water park, MGM or EPCOT). MK is almost always the one we do in the morning, but we will go to MGM and EPCOT at least one morning to do the "big" rides (TOT, R&RC, TT, and MS).
We always bring Brita water bottles from home and a mister fan (they have these available at WDW, but they cost more).
We never had to wait long at any restaurants either. I did make PS's to 50's Primetime, Hollywood and Vine Fantasmic Dinner Package, and Sci-Fi, but not until we arrived at our resort the day of our arrival on July 12th and I had no trouble at all getting any PS for the days I selected. We also ate at the RFC at DTD without a PS at dinner and only waited about 15 minutes. We never had long waits in counter service restaurants either.
If you like being around people (I do, it kind of goes with teaching!) you'll love summer in Disney. Everyone is usually very nice!
