Alice Sebold nicely marks the answer to your question by a line that vividly stands out in the text: "These were my dreams on Earth."
Namely, Susie's dreams on earth often focus on high school. Susie imagined herself at Fairfax High. (I guess it should be noted that where Susie lives and in her time, middle school involves 7th-9th grades, high school only 10th-12th.) Susie mentions that, when she got to high school, she would insist on being called "Suzanne." Susie goes even further about her earthly dreams:
I would wear my hair feathered or up in a bun. I would have a body that the boys wanted and the girls envied, but I'd be so nice on top of it all that they would feel too guilty to do anything but worship me. I liked to think of myself--having reached a sort of queenly status--as protecting misfit kids in the cafeteria. (16-17)
As her dreams go on, they become a bit more idealistic with Susie overtaking high school "in a matter of days" and ending with Susie earning "an Oscar for Best Actress my junior year." Generally, though, they are simple dreams. They are normal dreams. They are dreams snuffed out in one brief moment.
Susie's dreams in her heaven exist on two levels. First are the dreams that become real. Susie says it best: "We had been given, in our heavens, our simplest dreams" (18). The high school had no teachers. Susie only had to attend art class. The books at the school were only teen and fashion magazines.
As Susie learns more her "heaven expanded" to include more of her dreams becoming real.
Our heaven had an ice cream shop where, when you asked for peppermint stick ice cream, no one ever said, "It's seasonal"; it had a newspaper where our pictures appeared a lot and made us look important; it had real men in it and beautiful women too, because Holly and I were devoted to fashion magazines. (20)
But this section ends with the dreams that Susie CAN'T have in her heaven. She comes to a sort of epiphany here:
I could not have what I wanted most: Mr. Harvey dead and me living. Heaven wasn't perfect. But I came to believe that if I watched closely, and desired, I might change the lives of those I loved on Earth. (20)
Ah, . . . and it is this last sentence that clenches the dreams-becoming-reality of the rest of this wonderful novel, The Lovely Bones.