Maybe have it student-centered. Have the kids show their parents the ways they use technology during the school day and showcase projects that the students have created.
I agree with Marie. Our most successful open houses have included "student showcases" where each child chooses the software or project that he or she was most interested in during the year. On open house night, individual students became experts and even created "advertisements" using Pages (posters, flyers, etc.) to post on their desks to draw parent and guest attention.