These open houses can either be very simple or quite elaborate.
Some ideas:
visit the $ store for plastic tablecloths, party decor, etc. Your area's school colors will go quickly though.
If you have cold salads, meat tray, etc - purchase a shallow plastic under-the-bed box to put on the table. Fill it with ice, cover with colorful netting, and then place your bowls so they are nestled in the ice.
If you are doing sandwiches, you can go to Sam's Club and buy a big thing of ham and/or turkey (like they use at deli's) and ask them to fluff/shave it for free. Much cheaper than purchasing the meat at a deli.
Don't think you have to have lots of choices. In our area, there are typically lots of open houses going on and people tend to just stop in for a bit.
Do not serve alcohol .. this is your graduate's party and they are not old enough to drink. This also is just one less thing to worry about.
Either purchase can soda when it is on sale and then just have a big tub of ice with the soda in ... don't forget one for bottled water ..... OR purchase 2-ltr bottles and have ice and cups available.
Purchase a large decorated cake.
Try to have everything set out ... if you are using an outdoor area, I find it best to have the food in the kitchen just inside of the house but the beverages outside.
Don't hesitate to borrow seating/tables from the neighbors and/or your friends. Have the garage cleaned out in case of inclement weather so you can move the festivities inside.
Take lots of pictures!