Mostly, I just make cute signs in Powerpoint. When older S got his job with a financial company, I made him (in Powerpoint) a credit card/visa card looking thing with his name/fake account number/logo/amount. Then I did something similar with younger S, but his school logo was on the card. And then I had them laminated. And once opened, I transfer the $$$ into their accounts.
We’ve also given them one of those fake books that has a “secret compartment” inside and put $$$ in there.
Maybe buy some cheapie clear plastic piggy banks from the Dollar store and put the $$$ in there?
Edit: We also got a couple of gag gift boxes that says something cheesy like “hot lips” on it - and then we put something inside. I don’t think we put cash in it, but it was a bigger box. Maybe buy a box of candy and slip a check inside the top cover?