For most preschoolers there is only about 2 hours between the time they get to school and the time they eat, so really, they don't even need a freezer pack, but if having one would make you feel more comfortable, then get one.
Target usually has nice skinny ones in the $1 section during back to school time, in fact I think I just saw some there last week. Or you can get a harder plastic one, usually shaped like something, for a few dollars in the lunchbox section.
For my presschooler she is not allowed to have nut products at school so I usually pack a mini bagel with either cream cheese or jelly. I put little bowls of raisins, dried fruits, crackers, string cheese, yogurt cups, veggie booty, etc.
Other main course options are cold grilled cheese, jelly sandwiches, hummus and veggies, lunch meat, cold mac and cheese, and cottage cheese.
I rarely put a cold pack in my daughter's lunch and I haven't had a problem yet.