When you're bored and looking for a quick brainless activity online, games are a good first choice. Even though you may already have your favorites here are some of the things I like to do. Perhaps it will spark a new favorite for you.
1. Logic Puzzles. Ok, this one isn't brainless, but what used to require a piece of paper and pen and can now be done online. You can click the little boxes to denote yes or no answers and gradually figure out the solution. Puzzles ranging from easy to difficult are available and are free to play online. puzzlersparadise.com is my favorite site.
2. Flash animated games are fun, such as jigsaw puzzles and chictionary. shockwave.com has quite a few that are fun to do. These require a computer that can handle flash animation so it should have some speed, memory and power and a decent highspeed internet connection.
3. For the kids you should check out kidsgames.org. They're best for kids under 10 and will give them a great way to unwind and use their minds without being overly taxing.
Sometimes you'll find PC versions of the games you're playing and of course there are always ads to get through. Usually the PC version costs money so if you're looking for a quick free thrill then just use the web only version and be happy with that. Some games have versions that play for real money, but most of the time you also have to input real money from a credit card first. This can turn into a gambling habit so I recommend sticking to the free ones.