My friend has invited my boyfriend to her party. I need to get him away from that party! Any suggestions?

I am 12. My friend, Shannon, has invited my boyfriend, Justin, to her summer pool party on the last day of sixth grade. I'm going to a different middle school than the both of them. My mom and I have thought about planning something to do that day with him, so he doesn't go to the party. Another bad thing, Shannon has a mansion, and I don't, so Justin is even more tempted to go to her party. I need something to do with him that is SUPER fun, but doesn't cost a lot. I went laser tagging last night with Justin, Shannon, and other people. So, I can't do that. Anyways, the most fun event you give me will become best answer! Please and thank you for the ideas!