Great America is my home park and I can tell you anything you want.
The park's not open on the 18th(Physics Day), but since the 19th is a Saturday, you will have lines. Usually GA doesn not have lines, but Saturdays are definitely the worst.
Coasters: (clockwise as you enter the park)
Top Gun: Popular, but handles crowds well. Has one of the best lines in the park (not saying much). Expect lines around 30-45 minutes except in the early morning.
Psycho Mouse: Popular because it appeals to the families (the park's target audience). Add to that a low capacity and you get lines of 30 minutes almost all day.
Grizzly: Only has a long line if 1 train is running. Otherwise waits are generally under 20 minutes.
Invertigo: Long lines from opening to closing because of low capacity. Definitely ride this one early. Weekend lines can get up to an hour.
Demon: Sometimes has long lines, sometimes not. If you're already over in that area riding Invertigo early best to hit this one too.
Vortex: Waits are generally average, ranging from about 20-45 minutes. Since this is the closest roller coaster to the entrance, people rush to it first, resulting in long lines early. Later in the day the ride should have manageable waits.
Other (important) Attractions:
Drop Zone: Lines are generally the same throughout the day around a half an hour depending on how many sides of the tower are working. A good way to ride without much a a line is to return after a (frequent) breakdown.
Survivor: The newest ride in the park so lines are ALWAYS long. Hit this one as early as possible after Invertigo.
Action FX Theatre: Starts to gain a line around noon. Don't wait more than 1 show if at all.
Boomerang Bay: Since it'll be May, the waterpark will be open and boy is it popular! Your best bet if you want to ride any water slides (which there are a few good ones) is to go right at waterpark opening (11:00) and usually you can get everything done in a couple hours. Later in the day as it gets warmer, the water park fills up fast.
Any other rides will have waits of 1 or 2 ride cycles. Depending on the weather, the water rides could be pretty crowded too.
Good places to eat: Nicktoon's Cafe, Food Festival (food court that has EVERYTHING), Maggie Browns (fried chicken mmmmmmm), Dundee's Outback Shack (if you're near the waterpark). All will be very crowded during meal times.
Get there early and get in line for Either Invertigo, Vortex or Top Gun first thing. Then make you way around the park counter-clockwise or clockwise if the water park is important.
Hope that helps.
