One of the hot games of 2012 and 2013 was Spelunky, a platform game for the PC (and other platforms too) that provides an endless set of challenges. You can buy it in shops, or online, and it costs around $15.
Spelunky is actually based on a free game that came out a few years ago. The concept is similar, although the graphics in the latest version are far better than in the original freebie.
Spelunky is a platform game, based around exploration of caves. There are treasures to find, baddies to fight, and all the stuff you'd expect from such a game. And the soundtrack music is quite fun too.
You can get the original from http://www.spelunkyworld.com/original.html as a download for your PC. The file is 10 MB, and it's malware-free according to VirusTotal and Web of Trust. It's portable, so you can play it straight away without needing to install it. And it even comes with source code if you're interested in seeing how it all works.