Here is a great site for keeping the mind stretched whilst passing the time.

www.brainbashers.com  is a terrific collection of thousands puzzles, games and optical illusions. It claims to be the world's most popular puzzle website, and who am I to disagree? It is regularly updated with illusions and games and has five new puzzles added each week.

What's more, it's British; it is run by Kevin Stone, a puzzle enthusiast and theoretical mathematician at Keele University. 
He started it in 1997; initially it was as an exercise to see how much interest there was in an Internet based puzzle site, and it has grown since then.

It is now one of the busiest sites on the Internet, perhaps because he charges nothing to view the whole site. He does
gather some income from advertising, but uniquely (in my experience) you can turn the adverts off. Fantastic.

If you enjoy solving puzzles, crosswords and the like, you'll like this site. You can subscribe to a daily puzzle, too.