High School Cryptography

Hi all! I know this is probably pretty rudimentary compared to most topics that float around on this sub, but I teach AP Cybersecurity and I’m wanting to start a Cryptography Club at my school. It will meet once a month for 48 minutes. I want students to have fun, learn, and want to recruit their friends to the club in hopes of eventually piquing their interest in AP Cybersecurity. I need ideas on what to do during this club time to keep students engaged and excited about coming back. Do any of you have any awesome cryptography lessons you’ve been involved in or created over the years you’d like to share? I’m looking for things beyond a Caesar Cipher (I’ve already got materials for this) but not so complex that I’ll lose student interest. TL:DR; Fellow mathematicians, help me remember some of the cool cryptography things we learned in college that high school kids would find cool. TIA.