I live in Canada and I did this for my daughter who turned 11 in October. The theme of this party would also work well for your age group too.
My daughter figured out who she wanted to invite and then told them ( all girls, could work with boys as well ) the party was on a Sunday afternoon. Come over to the house and hang out, etc. The flip side to this was instead of doing a surprise for my daughter, we actually surprised the guests instead. This is where the parents of the people you invite need to come into the picture. Mum's the word though!! Let the parents know that as far as the guests are concerned the party is on the Sunday, yet actually you will be coming by the house early, early on Saturday morning and waking up your friends from their slumber. We went to the first house at about 0700 and then carried on from there. We went into the house with video camera and one of those instant cameras that you can take right in to get processed. We videoed going into the house, to the room and inside the room and waking up my daughter's friend from their deep sleep. It was so funny to see their reactions waking up. We ended up going to 6 houses in total, with my sister following in another car behind us. We set up a scavenger hunt for the car while going house to house and had chocolate coins and little dollar store items for prizes. As we went into each house so did every one who was with us. By the time we got to the last few houses, all the girls were laughing constantly and having a blast.
We got back to the house with all the girls. Oh yea, they have to remain in their P.J.'s for the party. Get the parents to secretly pack clothing they can change into after the party is over. When we got back to the house, my husband had made a fabulous breakfast of pancakes with strawberries whipped cream, chocolate sauce, bacon, toast, eggs, juice, etc.
During this whole time we had 2 of the disposable cameras and were taking pictures of all the girls at various stages of the beginning of the party. While the girls were having breakfast, my sister took the cameras to a place to get developed in 1 hour. Ask for double prints so that there are enough pictures for everyone. I picked up frames from the dollar store. Outline the size of the actual picture area on enough contruction paper for everyone. After breakfast spread out the pictures, glue, scissors, crayons, glitter glue, stickers, whatever and let your friends go wild making a momento of their day at your birthday party. This went over very well!! We then played a bunch of games. I.E. - candy on a paper plate, everyone in front of a plate hands behind backs, spray whipped cream over candy and the winner is the one who gets all the candy first. Made for great pictures!!
Once the games are over, have the guests get dressed and sit them down to watch the video tape of them being woken up. The girls loved this part. Lots of laughter.
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