From our experience from our just returned 10 day trip:
We went to HS, AK, MK three times each and Epcot once and just hopped to Epcot other times.
We were at the Grand Floridian. We arrived at the bus stop for HS and AK 90 minutes before opening. We got on the first bus each time.
AK was no problem, there werent many people on our bus each morning and the driver arrived around 6:50am for a 8am open,got us there by about 7:05 maybe and out of the 3 times we went 2 of them we were the 3rd and 4th people in the entire park. They just let us in early, didnt even wait until 7:15. A cast member escorted us to FOP. The third time we got there about 7:15am and we still walked the entire FOP queue and ended up being loaded on the first ride of the day.
HS we were soooo glad we arrived 90 minutes early, 7:30 for a 9 opening. We were about the 3rd couple at the bus stop each time and got on the first bus each time but by about 7:50 the line was so long i have no idea how long they had to wait for other busses. 1 out of the 3 times we were at the front of the pack and rode Slinky Dog, TOT twice, RNRC and Millenium Falcon before the crowd started to increase. The other two times we were in the opening fray but got in quickly and could still ride TOT twice and RNRC and MMRR and Toy Story Mania before it started to build.
As for MK that was easy, we walked once and took the boat twice, got there around 7:00 for an 8 am official opening and were the first people to be held at the front of the walkway or dock and they let us all in around 7:15 and before any TTC boat even was in sight or had arrived yet. Since they held us much closer to the entrance then the bus people, we were thru and some of the first in line at SDMT or BTM or Splash, but they didnt let us start riding until 7:45-7:50am. The only rides i saw operating before then was the Carousel and the Magic Carpets. After our first ride, BTM was literally walk on for the first couple of hours, same with Haunted Mansion and Pirates only had a small wait. The lines that started to build were of course SDMT, SM, Splash and Jungle Cruise.
We usually left around 1pm and returned around 4pm and either went back to the same park or a different park because the wait times went down and lots of people were leaving.
So long winded post, but our best advice, if you can get up early, is to be there before opening, you can ride quite a bit for the first 2-3 hours or so with smaller lines, then leave when it gets crazy and go back for the evening.