There are a number of great themes that you can do for your grand daughters party. Such as a sea life themed party, a pool side party, a beach party, a fairy tale themed party etc. However it would be best if you keep in view her likes and dislikes and choose according to it. Here is a link for great ideas:
Have you gone to www.theparty.net yet? You can choose Graduation Party and then choose everything you need and the zip code and it gives you all the companies that fit your needs. Worked for me!
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.