1. Although I think whatever is on people's "must do" list for US will be highly subjective depending upon what they like and enjoy the most; I would say this park's "must do/see" things are Kongfrontation, Jaws, Earthquake, Back to the Future, E.T., Terminator 2 3-D, MIB, Nickelodeon, I Love Lucy Tribute, and Twister
2. If you only got to see a few attractions during your first trip, I would suggest to definitely have a rough itenirary of the attractions that you may want to see next time.
Also, it might be helpful to familiarize yourself with the park as much as possible such as looking at a map and finding out beforehand where all the attractions you're interested in are generally located so you won't waste a lot of time (especially if you have a limited amount) wandering around the park looking for things.
Additionally, I would pretty much arrive early to get a jumpstart on the day particulary if you are visiting during the busy summer season with lots of people there. Also be sure and take time out to review the park brochure once arriving that's given as you pass through the entrance to get an idea of what's currently offered as well.
3. As far as the best time to go on rides, I have found that during the park's opening the most popular and well known ride lines can grow long quite fast. However, I usually check these first because sometimes you can get a lucky break and get a chance to be one of the first persons in line.
Also, during the traditional lunch times between 12-2pm or so I have noticed that a lot of people choose to break for lunch, midday snack or rest so you may be able to experience somewhat of a reduction in wait times then.
At the end of the night or whenever closing is drawing near a lot of people are really tired and usually dragging their feet from touring all day so this time may be also conducive in getting on some of the rides you may want to go on without having a long wait.
Hope this helps some and you enjoy your next trip.