Wow, I am amazed by the people on here who can just go, go, go at the parks. We can do a long all day type scedhule, but the problem is that it usually kills us for the next day. So we try to be very careful since we feel fine in the park and think, "Let's keep going!" since it could mean that we miss something else the next day. Knowing this, I do try to plan a rest morning the next day after a long park day.
Last trip three weeks ago we did stay in Epcot all day...from opening until close with 4 kids ages 1, 6, 11, and 12. Two things made this doable...one, there were no crowds. Sometimes I need or want a break from the parks just to get away from the crowds. And two, we spent the whole afternoon in innoventions...an area we had skipped all other trips, and we just wondered and poked and dawdled and explored. DD1 fell asleep in her stroller, so I skipped going back to the room with her as was planned. Those hours in innoventions were like a break for us.
We have done a similar break at MK on Tom Sawyer's island on an all day park day, and that worked well also.