we use them for allowance for dd to help her save spending money for the upcomig trip...she gets half her allowance in cash and the other in DD. We also put them in birthday cards, xmas stockings, easter eggs, etc. They are sooo cute, my favorite right now is the $10 with Tinkerbelle on it.
DH and I even turn rebates/rewards that can't be used at DW on their own (like a Mastercard giftcard that will expire before the trip) into DD so we can't spend it on something else.
Kids get a real kick out of them. Putting some in dd's copy of 'Kingdom Hearts' (a new disney/fantasy role playing game for ps2) for xmas. And some in the stocking, too.