If you are looking for impromptu games that can be organized in less than two minutes, consider line dances like the Charleston, Jitterbug Stroll, the Fuji Apple, the Big Apple, the Shim Sham, or the Madison. The Steal Dance (where one person tries to "cut in" on a dancing couple without interrupting the flow of the dance) also can be fun.
One dance that gets everyone involved is the Snowball. This dance starts with the majority of people forming a circle around a few dancers (maybe four or six) in the middle. When the music begins, only the people in the middle dance. A little while later, the party planner or DJ calls out "Snowball!" That's the signal for each of the dancers in the middle to grab a new partner from the bigger, outside ring. This process repeats until everyone is dancing.
Another fun dance is a restriction dance where someone calls out a body part that can't be used like eyes, right hand or left foot. You can also form a jam circle. Just make sure there is a system for each person to have a turn, and everyone knows the signal for when his time is up. The best dances get people up and moving, so try making up games that encourage people to dance with others they don't know, such as ladies choice. A DJ challenge can bring a change of pace because it gets people dancing to a song they wouldn't normally dance to.
Some surefire ways to get people on their feet and moving are to do some dances that are always seem to be popular. One of these is YMCA. Not only will be people be dancing, they'll be singing along to the song at the tops of their lungs. Other oldies but goodies are the Charleston, Lindy Hop, Chicken Dance and the Hokey Pokey. People also seem to enjoy the Cha Cha Slide as well as Tte Chain Dance.
Having a dance contest inspires people to get involved. Be sure you have neutral judges and a fair system that allows everybody an equal chance to win. You also can can pick the winner by who gets the most applause. Choose a variety of categories for this contest. For instance, dancers could compete for coolest dancer, geekiest dance or smoothest dancer.
Another fun contest is one where dancing couples have balloons around their ankles and try to pop other people's balloons while they are dancing. The winner is the last couple to still have an unpopped balloon. Of course, any couples who aren't dancing during the contest should be disqualified.
The number of fun dance games you can come up with is only limited by your imagination, so let your brain boogie and put on your dancing shoes.