I've been a cashier just over 3 years now, and I've seen (and kept) more than my share of old bills. Generally, I won't keep anything unless it's from series '88 or earlier; series '96 $1 bills hit circulation hard every time somebody discovers an old change jar and the economy takes another dive.
But, I've had some interesting finds over the years, including:
Series '35-E (Priest/Humphrey) $1 silver certificate. Used to buy groceries.
Series '63 $10 star note. Used to buy a scratch-off lotto ticket. Still crisp.
$280 in series '81-85 $20 bills, all lightly circulated. The customer said that it was her "rainy day" fund from college, but she realized it was worth about half as much as it was back then.
And of course, more than $400 in star notes (about 80 bills, mostly singles) and a nice collection of neat serial numbers, including a $20 bill with serial number 00000211.
And truth be told, I will probably end up spending most of that collection. Bills are expensive.