This BusinessInsider article might be a useful activity to try in your classroom for a few weeks or even a month. Challenge your students with debit and/or credit cards to put them away and only use cash for a period of time. Probably best for them to estimate their weekly expenses and then take out that amount at the beginning of the week. While they spend, they should also be sure to collect receipts so they can track their spending. The research is well documented that we spend less when use cash instead of plastic.
Rather than tell your students about this research, why not let them experience it and then reflect on what they learned? You might also want to join your students and report back to them on your experience also.