I think the idea is neat, but a lot of drawbacks as you saw with the first response you got. Plus, would they be tempted to blow some of the cash on really dumb things?
How about a stack of monopoly or other play money totaling $100K, then an envelope in the bottom with the cashiers check?
Does Nestle still make the $100,000 bar? You could do something like wrapping that and giving it to them first, then the box with the play money, and then the check in an envelope or some other way.
Depending on your income level and theirs, it might be better to gift them appreciated stock if they can sell it with $0 cap gains. But I understand your wife might want a more direct correlation with the annuity from her father.