site59.com is worth a look. They sell as air + hotel or + car or both packages. It's the same engine as lastminutedeals.com or Travelocity's last minute packages. Some
extra $ off, maybe google for more.
The air/rental car packages are generally a better deal (often the site59 + car is < best airfare elsewhere). Also check betterbidding.com/biddingfortravel.com for hotel rooms. You should be able to get a 4* hotel in Chicago, StL or SF for <$100. Boston can be a bit more, and priceline isn't so good for Vegas.
I've been to all five cities, lived in two. Usually I suggest going someplace new. SF, btw, has a much lower key July 4th than the others. Chicago summer is more or less built around the 4th/Taste of Chicago, so it's a good bet if you like crowds and meat.
Enjoy.