Mastermind
Or in my case Mini- Mastermind.
I often hear comments along the lines of ' this is the board game where the box cover was more exciting than the game'. I would have to disagree , I think this is a great game for encouraging logical thinking. The rules are simple, one person sets the code the other tries to guess it. The code setter marks the guesser's answers using black and white pegs to show how many correct parts of the code have been guessed.
It's one of those great games for a wet Sunday afternoon, it takes just enough brain power to keep children interested but not so much that it become frustrating. Of course there is the full sized version of this board game as well. But I just had the mini-version or travel version, not sure on the reasoning for that as I certainly would never take it travelling (far too easy to lose pieces.)
Again it's still a game that I have in the attic, and it still seems to have all it's pieces. Not sure how I managed to keep all those together all these years.