Classic Games
Often the classic games which have been around for years can be the best spur of the moment choices. Enjoy a round of Pictionary, Twister or charades with little to no supplies needed to play. The traditional card games also can be played easily once everyone knows the rules. Try card games such as bridge, rummy and Texas hold 'em, which can be fun for everyone.
Depending on your space available you can get out your old set of dominoes and see what kinds of patterns and designs you and your friends can make together. If floor activity isn't easy for everyone you can also play guessing games which let you and your friends learn more about each other. Playing Two Truths and a Lie gives each person a chance to make three statements, one of which isn't true, and the rest of the group gets to guess the lie.
Board Games
If your friends would rather play board games, a world of possibilities is available. Anything from Monopoly, to Life, Scrabble, to Settlers of Catan can be played with a varying number of participants and can take a while to play. Choose something simple such as Checkers or Chinese Checkers, or pick a more strategic games like chess or Parcheesi. Newer games, such as Balderdash, Munchkin or Bang might take more time to become familiar with the rules but can be as entertaining as the classics.
Outdoor Games
Head outside, to a park or your backyard, and expand the game options even further. A round of croquet, badminton or volleyball are fun options for people who are seeking a little more physical activity. Outdoor games can help you and your friends to get a little leisurely exercise. If you have the space available you can also set up relay races through the yard with obstacles or challenges to be completed along the way.
If your having a pool party, have a game of Marco Polo or Over Under.