I like the spray foam idea. Fill a small bucket with spray foam and toss random amounts of money into it but layer it somehow. Make the outside edges look like its worth it with quarters or a few dollars maybe and make it worthless as you get more into it so they end up with $3.48 or something random.

One year my dad rolled up $1 bills really tight and small with packing tape. Threw them in a huge box full of green packing peanuts and said there was $50 in there but left it like $6 short or something. I went through the peanuts multiple times before I caught on that he might have just left some out to be even more of a pain.