Writing RPL on my HP48 as a young teenager influenced the way I thought about programming significantly. RPL IMO is underrated as an educational tool. It introduces the stack based concept, but also the "everything is an object" concept. As a useful language I'm not sure, but it was definitely fun to think about.
Not sure they are that interested in programming, but FORTH is a little sweet language.
My first experience with it was Paul Lutus’ GraFORTH. On the Apple II it was ridiculously fast.