Basically everyone has played it already but if you havnt plants vs zombies is pretty close to a perfect game, its just the perfect kill little time game.
http://store.steampowered.com/app/3590 5$
World of goo is as good as plants vs zombies but its less universally appealing as some might find it just little bit to puzzle like, still the sound and graphic is usually enough to convert even those who hate puzzle games.
It just oozes charm.
https://www.gog.com/game/world_of_goo 9$
Two tiny games which are brilliant their only drawback is how short a time it takes to finish them but there is no dead āfillerā time every minute you spend on them are fun.
If you want a bit of a bigger game that fills more time id suggest xcom: enemy unknown which is a old school turn based group shooter were you defend earth against ill let you guess what its not illegal immigrants hint.
I didnt think it sounded that fun when reading about it but actually playing it is so much fun its one of those game when your finished you almost want to cry, im never going to be able to find another game that good again were is my time machine or memory eraser machine when i need it!!!
http://store.steampowered.com/app/200510/ 19$
Think the downloadable extensions for this game are disappointing just not good value for money.