For zoos, the critters tend to be much more active during the early morning or early evening hours. Middle of the day is not that good a time to visit.
And remember, lines frequently get short near closng time if the park is closing late enough that day, especially on a Sunday.
Disney World however, I try to get there early since I know I will be waiting for a while.
Some theme parks lose their magic at night. Beside in night, you can't really see anything. Morning's air also tends to be much more fresh and healthy, thus you don't get tired that much.
At night, it gets creepy. You see a lot of people tired, oh, that really lets me down when I see people tired.
But I also often stay till closing, escpecially when on vacation where I use all available time to enjoy the park im at.The only time I dont stay till closing is when I go to my local SF park where I make a half day visit and get in alot rides and leave when it gets really bad in terms of crowds.
For my local park, World's of Fun, I do both as well. There are days where I plan to go to the park and get there before the gates are open, spend a few hours there and come home. Then there are other days when I am bored in an summer evening and just decide to go for a couple of hours before the park closes.
Do any of you do opening and closing but take a break in the middle of the day? During the summer it sure makes it easier.
Subsequent visits can be taken at a leisurely pace.
I can never seem to hold back my excitement when It's the day I go to an amusement park. That is the only time that I feel like waking up at 6:00 even though the park opens at around 10.