Some of my favorites so far in no particular order...
New Orleans (Fun, Historical buildings, Fun, Great Night Life, Fun)
Miami - South Beach area (Lots of great looking people and lots of money)
Kansas City, Kansas (Not your typical big city but a real gem in the heartland)
Palm Springs (not a big city but really a great place to relax)
Boston (Haven't been there in years however we are looking at going back in the next year or so. Again great history, seafood and just a fun place.)
Toronto, Canada (this is a great city, spectacular night life, shopping & something for everyone. Check out the underground shops and the gay area's.)
Vancouver, Canada (We were blown away by this spectacular city. Like Chicago and NYC it has various neighborhoods with lots to offer whether you are visiting for a weekend or a week!)
If you are looking to travel a greater distance...
Tokyo (This is an amazing city that has a little of everything. If you are looking for a modern, culturally diverse, international food and great architecture this is a must visit.)