my answer... yes, make plans! If it's a slumber party, food - likely dinner and breakfast - will be a definite. My twin daughters had a slumber party two years ago for their tenth birthday, and though there is a three year age difference, a few of our ideas may be helpful:
-dinner: I had mini pizza crusts and let each girl put on her own toppings. We placed these on cookie sheets with their initials on the foil, so I knew which belonged to which girl. With breadsticks, this was our main supper. Dessert was birthday cake.
-breakfast: I had clear cups and plastic spoons for the girls, then I had bowls out of cut up fruit, cereal (granola or cheerios) and yogurt. They made little "breakfast parfaits," so they got to be fancy. :)
-games: if you have a dollar store nearby, or at a Wal-Mart party supply area, pick up scads of prizes. Then, whatever game(s) you play, the girls can get a prize when they win. (We had a limit of two prizes per girl, but they could still play... we had six girls and had 12 prizes.) One version of a party game we did at our girls' sixth birthday party was "tape the jewel on the crown." Rather than a donkey who needed a tail pinned on, I drew a large crown on a piece of yellow posterboard, then had construction paper "jewels" that blindfolded girls could each try to tape on the crowns top. Those whose jewels were closest won. Or there is musical chairs - that's a fun, simple game.
-crafts: I suggest you have something little, even a birthday picture for them to color... my girls always loved stringing beads, and "pony beads" can be strung on yarn, for instance.
Since it looks like the weather will be nice this weekend (at least in Indiana), if you have a backyard or park nearby, outside activities abound! Girls would enjoy making chalk drawings, I'm sure, if you have a paved/concrete area for this. Another fun outdoor thing, in nice weather is blowing bubbles!! Or playing with balloons. :)
I obviously wouldn't do all of this in one slumber party!! I just know that having some things planned can help. And I definitely suggest that you have a rated-G movie on hand for the girls in the evening. Popcorn is good for movie time, and it's not too expensive.
Have fun!
One final thing that comes to mind is dress-up... if your daughter has fun dress-up clothes, the girls can create all kinds of things.