You could make (Her) the theme of the party, by getting together pictures of her from the time she was a baby and on up through the years. Then put them everywhere.
You can also make a bullitin board with all her school year pictures. Maybe have a couple of the special ones blown up to poster size.
You can gather up all sorts of things from her growing up years, such as her baby shoes, craft projects and finger paintings, a letter she wrote to the Easter
Bunny or Santa. Some baby clothes and shoes, her favorite flowers. Hang and display things all around.
Then, all her friends and family can take turns telling funny stories about her.
Maybe some of your grandaughters favorite things. What does she like?
Anyway, it's just a suggestion. Your grandaughter is a lucky young lady to have a grandmother who is already thinking about her 8th grade graduation. I'm sure that whatever her Nana does, she is going to love it.