Sudoku
Most people find Sudoku a fun and addictive game, plus it can help improve your problem-solving skills.
Wikipedia:Random
Wikipedia:Random is simply a means of randomly stumbling on a Wikipedia article. Learning something new every day can keep your brain healthy.
Practice simple math
A minute of simple math can help get your brain in shape and make you far less reliant on a calculator.
Lumosity
Lumosity is a webapp that provides specialized brain-training activities. All the exercises are pretty simple to understand and are fun to play.
