What age group are we talking about? And is this for just girls or boys, too? How many friends?
EDIT: ok, thanks for the info. Sleepovers are always good and memorable because you get to spend all that time together to make a great memory. Having guests bring plain pillowcase (with Mom's approval) and having everyone sign and write something on each others' cases using colored permanent markers. (Be sure to insert a cardboard in pillow before writing so it doesn't bleed through).
Serve your favorite foods and snacks...here's a festive treat: Cake Poppers>>
http://www.bettycrocker.com/recipes/cupcake-poppers/c0d659b8-4c2f-41ec-a080-a78af7ef5f20?WT.dcsvid=MjA5MzYzMTcyNgS2&rvrin=3A5566DF-81B7-4C7E-9206-9C28699C6171&WT.mc_id=Newsletter_BC_03_24_2011_Bi-Monthly
You can make quesidillas...nachos, dips and chips, fruit kabobs to dip into canned chocolate fudge frosting that's heated up in microwave safe bowl on high for 20 seconds. (Make brownies, cool, then put in freezer for 20 minutes before cutting into bite-size pieces. Alternate fruit like strawberry slice, brownie bite, green grape, brownie bite on skewer and dip into the easy "chocolate fondue").
Watch movies, create a scrapbook for her (include everyone's name, address, email address, phone numbers, cell phone numbers, etc)...take lots of pics, have a White Elephant game....
http://www.associatedcontent.com/article/490899/how_to_host_a_white_elephant_gift_exchange.html
Decorate with a few balloons, streamers, a banner wishing her good luck.