Lots of good advice so far! I'll try to add a couple of items.
At MK, a great spot to watch the parade and is in front of the castle - you can get pictures with the castle in the background. You will need to arrive early though. Also, in Frontierland, across the street from the Liberty Tree Tavern is a good spot. I like this spot, because you can exit easily through Fantasyland.
Same thing for the fireworks, you will want to get to a spot on Main Street really early. If you don't want to do that, you could watch the fireworks from the Ticket and Transportation Center. If you are heading out anyways, this gives you a "jump" on the other guests leaving. You can also walk from the TTC to the Poly resort and watch from the beach - more romantic - you will still get the music!
We have stayed offsite, and we stayed all day until midnight - wouldn't even have considered making the trip back and forth due to losing valuable tour time! We would use the time waiting for the afternoon parade (at least 1 hour) to relax, eating a Dole whip or hot dog from Casey's. (Dole whips are available in Adventureland and Casey's is on Main Street) One person would sit and save two seats while the other person makes food runs.
At Epcot, Illuminations is good from Italy also, there are benches (facing away from the lagoon, but at least a place to sit while you wait, and you can just flip around to kneel on the benches when the fireworks begin). We did the same thing here with letting one person sit and the other person go get snacks. Try the beavertails from Canada or anything from the France bakery.
To view the parade at AK, at the time everyone else is lining up, go to ride the safari, when you come out of the ride, the parade should be heading back (it circles around and ends where it begins) you still are able to see the parade, but without the long wait time.
MGM, I would suggest the dinner package for Fantasmic (need to go ahead and do PS's) because you get a spot easier than waiting for 1-2 hours. This is also a way to get a good sit-down meal, and you can eat more leisurely.
Use the various "shows" to relax in a cooler spot during the hot part of the day. For ex. MK - Philharmagic is great way to sit and relax, even the Hall of Presidents is an option for the middle of the hot afternoon. At Epcot, in the American Pavilion, you have a longish show to sit and relax you can also go to Honey, I shrunk the audience. Many of the films in Epcot are standing and so not necessarily a choice to sit down in. At AK you can sit and cool off in It's Tough to be a Bug and at MGM, you have Little Mermaid and Who Wants to be a Millionaire.
Decide if you have must-rides and try to do them early or very late. We always rode Space Mountain at about midnight as a walk-on! Until the final minute, the ride is still open, there were many times they would let us in and then the rope went up behind us to close off the riders. So, if you are there at the end, head to your favorite ride.