Ok, So the licence is CC0, so its free to use and you dont need to credit the author if you want. I think, this is a nice level http://www.pasteall.org/pic/show.php?id=78372
in level > enviro_super_test.blend and with a combination of of the character, that can walk and shoot could be interesting little demo. There are even some monsters as you can play in the demo, shooting them.