You probably already have a pack of playing cards. If you don't then you can buy a pack for around $1 pretty easily at most department stores.
There are countless card games you can play for hours upon hours of entertainment.
Bridge is an elaborate game for 4 players.
Poker has grown in popularity in the past 10 years and has always been one of my favorites.
Go Fish and War are popular games for children
With Solitaire games you can play alone. Solitaire Central has rules for 1500 varieties of Solitaire.