Originally Posted by jpdx
Not easy to find these deals; often last minute offers on Travelocity etc can be pretty decent value, if you add a cheap car or hotel room.
Agreed that the Site59-type deals can be great at the last minute, but the OP is looking for last-minute day trips. Site59 packages usually have a two-night minimum stay.
Last-minute web fares offered by each airline (E-Fare, NetSaaver, etc.) can be reasonable, but these tend to apply only to nonstops and require an overnight stay. Two strikes against a good MR.