I've put together a fun little single player for Half-Life 1 called Mario Keys. The whole mod is built using Mario textures, sounds, and some custom models.
The premise of the mod is that Mario again has to save the Princess from the evil Bowser. You'll need to find color coded keys to access new areas.
There is very little combat, mostly exploration and puzzles. You'll also need to be very good with the long jump.
You can download the mod here:
http://docrocksmaps.homestead.com/Mario_Keys_non_steam.zip
I'd like to hear what you have to say about this mod. I made it all strictly for fun with the younger Player in mind.