I have always liked doing logic puzzles. Since they became popular this has included the Japanese puzzles such as Sudoku.
There are now quite a few types of these puzzles and I regularly complete them online. If you have never tried them why not have a go. A selection of them are available on this site courtesy of www.conceptispuzzles.com. If you have never done logic puzzles before a tutorial page is also provided.
I have also added some free puzzles from other sources and some Strategy Games.