If it has to be in the U.S.
I would say the best time to travel would be after Labor Day and before Halloween (If you want good weather in the big cities).
If weather is not a major concern then you're bound to get good rates After Labor Day leading up to Thanksgiving and again after New Year's Day leading up to but before "Spring Break".
This list is in no particular order (you can research for activities)
Maui Hawaii (Stay on West side of island near Kaanapali)
Party on Front Street, Haleakala Sunrise Tour, road to Hanna
New York City (Manhattan/Time Square)
Chicago (Stay within walking distance of Magnificent Mile)
Los Angeles/Hollywood/Orange County (Beach cities)
New Orleans (great food and a party every night on Bourbon St)
Las Vegas (if you can afford the shows, gambling gets old)
Miami/South Beach and Key West in the Florida keys
Seattle, WA ( downtown (Westin/W hotel) near monorail.
Party in Pioneer Square. One fee gets you into several night spots. Also take the underground tour, Space Needle...etc
San Francisco, CA
Most times it's cheaper to fly into Oakland and take Bart to downtown San Francisco. (note Bart is a subway train which travels in a tunnel underneath the water from Oakland to Frisco)
I like staying at the Mariott downtown next to The Metron movie theater. It's centally located and you can take the cable cars or taxis to various points of interest,Fisherman's Wharf, Pier 39, Lombard St. take boat to tour the bay or Alcatraz
Dallas or Austin, TX
From what I've heard there's lots of things to do in either town.
However I've yet to visit them.
If you have a passport it might be worth it for you to check out rates on apple vacations.com from time to time.
They often have great good prices to all inclusive resorts in Cancun and other tropical islands which may work out to being cheaper than buying meals, alcohol, and paying for transportation. Once you check into these resorts you have everything you could want (beaches, nightly entertainment, food and alcohol 24/7 and if you have extra money you may wish to parasail, zipline, or do some other activities..etc)