As my kids keep growing (damn food!) I keep introducing them to games I used to play and love. Most of them I find on GoG but I do have my other sources too. :)
One of those old favorites was definitely Blizzard Vikings. It is a platform game with you in a role of the three vikings taken from their home by evil aliens. Fortunately they got out of jail and through many levels they will try to get home.
To make things interesting, each of them has their special skill. Eric is fast, Baleog has weapons, and Olaf can take care of defense. Only by using each of them you can pass the conundrums in front of them. While first few levels are tutorial and they aren't hard at all, further you go the more of your gray matter has to be used.
Keyboard controls are probably the worst part of the game. They have to be learned and they are not intuitive at all. Yes, first few levels will teach you everything you need but if you leave the game alone for a few days you are back to square one. Cheat sheet is mandatory.
Of course, it is noticeable that this is an old game. Everything is blocky and resolution is as low as it gets. However, puzzles are interesting enough that you don't care much. My kids (8 and 6 years) spent hours playing this game and not in one moment they mentioned anything was wrong with graphics. Yes, they do notice it is "different" but they didn't see it as a negative thing.
Best of all, I found Vikings on battle.net as a free download. Thank you Blizzard.