Our Open House is also held before school starts, but ours is more of a parent information night.
Here are some ideas~
scavenger hunt-the students need to find some items in the classroom that maybe you can have set up and a short explaination of them.
back to school survival kit-students and parents need to come up with why each item is included in a bag. There could be a small prize if they come up with a reasonable solution. If y ou need items for a back to school survival kit there have been some posts about them.
if you have access to a computer you could have a mini Power point presentation going on about your grade level and you.
These are just some ideas.