As far as other games go that are free online, there are plenty of options. Check out Set, KenKen, Sudoku, Vertex, Tiles, and 2048.
Duolingo works. It's proven with learners at scale. But I worry branching that out to classical subjects like bio and math just reinforce rote memorization.
Memory games never work for me outside the virtual world.
I think turn based strategy (think GBA Advance Wars) games have the best shot at improving young minds. Teach them how to plan ahead, in environments with complex branching factors and every thing else will work itself out.