Are you only going to IOA, or are you visiting both parks? Some of the things previous posters have mentioned (Horror Makeup Show, Beetlejuice, Rip Ride Rockit) are at Universal Studios. WWoHP is at Islands of Adventure.
If you can choose your day, Tuesdays, Wednesdays, and Sundays tend to be the least busy - but during spring break it will probably be busy most days. WWoHP is the newest and most popular area in either park, there will be crowds and lines any time that you go.
Try to get to the park as early as you can and check out WWoHP first thing. Sometimes they open the park 15-20 minutes earlier than the posted time, if you are at the gate you can get in early. Most people try to do Ollivander's first, if the line isn't too long, and then the Forbidden Journey Ride. If you don't get to do one of these in the morning, try again in the late afternoon/evening, crowds may be lower then.
Tip about the Ollivander's show: You can watch it on
youtube. Check it out and decide how long you're willing to wait in line to see it. It is cute, but very short - some people are disappointed to see a 5 minute show after waiting 45-60 minutes. And even if your children are selected, you still have to pay for the wand if you want to keep it.
You can also purchase the wands at a cart near the Hippogriff ride, and at other gift shops throughout the park, if the store is too crowded.
At Forbidden Journey: If you are taking a backpack, camera, or anything else that does not fit in your pockets, you will be asked to put it in a locker during the ride. The lockers are free for the duration of the ride.
Youtube also has videos of this, I think - although some are dark, you should get an idea of the movement of the ride and if you think it will be too wild or scary for your group. There are spiders, a dragon, and dementors on the ride. Riders must be at least 48" tall.
Forbidden Journey does not use the Universal Express Pass, everyone must wait in the same line (with the exception of single riders). The single rider line tends to be much shorter, but you do skip most of the regular line, which is a very nice tour of Hogwarts castle with a lot to see. During the ride, you really can't see the other people in your car, so you might think about whether you want to do the single rider line or not. If you do, they usually also offer a "castle tour" line which will let you see most of what you missed by doing the single rider line.
There are two other rides in WWoHP: Flight of the Hippogriff, a calm family coaster, and Dragon Challenge, a "big" inverted coaster with loops, a big drop, and 55+ mph speeds. Dragon Challenge riders must be 54" tall, and this is also a "hands free" ride with free lockers available. You can see Hagrid's Hut from the Hippogriff line, and the Weasley's car stuck in a tree from the Dragon line - both have "chicken" exits if you don't want to ride.
You might also find out the times for the Hogwarts Frog Chorus and the Triwizard champions, two fun outdoor shows that you can catch several times per day. The kids can have their picture taken with the stars after these shows.
You can eat at the Three Broomsticks restaurant, if you wish. It is counter service, with seating available, for lunch and dinner. There is a sticky on this board that shows the menu and prices. It is full service at breakfast; they do not always have walk-ups available for breakfast, but you can ask when you get there. There are lots of other great places in the park to eat, if you'd prefer something else.
You might also wish to walk into the restrooms, if only to hear Moaning Myrtle. And look in all of the shop windows. The ones with actual stores are small and crowded, but it is fun to see what's in them if you get a chance.
There are two kinds of butterbeer - regular and frozen. I prefer the frozen, but both are good. The pear cider in WWoHP is very good as well. If you wish to purchase adult beverages, they are available in the Hog's Head Pub (which also serves non-alcoholic pear and apple cider and butterbeer).
A few people mentioned Universal's Express Pass, which allows you to skip the regular lines for many attractions. This can be a good idea if the park is busy, but do not buy it in advance, as it is date-specific and non-refundable. If you get there and it is very crowded, you can go into nearly any store in the park and buy them. Two things to note: they are only good once at each partcipating attraction, and there are several attractions that do not participate (Ollivanders, Forbidden Journey, Pteradon Flyers, and Rip Ride Rockit.
The rest of IOA is great too - my favorites were The Hulk roller coaster, and Suessland. There are also several water rides - but beware you will get a little wet on Jurassic Park and completely soaked on the others.
If you get a chance to check out the Universal Studios, there's lots of good stuff there too - including more coasters (Rip Ride Rockit, Revenge of the Mummy), a simulator (the Simpsons), Men In Black (where you get to shoot aliens), and a lot of fun shows. You will need a 2-park ticket if you plan to check out the studios side.