I'm getting a new game, but which one?

So today I babysat my 7 and 5 year old cousins, and now my mom's saying as a reward, I get any game at the price cap at £50 (~$78). What game should I get? [FROM STEAM]

NOT;- any CoDs or Battlefields.

I am open to Indie titles.

EDIT: I took out MMO from the NOT list.