Please, whatever you do, don't ask "So, what are your plans?" Here are better, less stressful ways to make small talk with the graduate and their family.
"Congratulations! How good does it feel to never have to take another final exam again?"
"So, what are you most excited to do now that you have some free time this summer?"
"Looking back, what was the best class you took, or the best part about your time there?"
"You must be so proud today. How are you holding up with all the excitement?"
"Well I know you have a ton of people to talk to, but I just wanted to say a huge congratulations. I'm so happy for you!"
Because every single person at the party will ask them that question. If they don’t have a job yet, it causes anxiety. If they do, they are tired of repeating the same pitch. Ask about their summer or their favorite memories instead.
Usually 1 to 2 hours is perfect for an open house format. Make sure you greet the graduate and their parents, grab some food, and then you are free to leave.