Christmas Bonus SKill!
Well, that was fun! Enjoyed it much more than the Idioms!
Just an idea, but I think it might be better to push the option to pick up those units farther along in the tree...? There is actually quite a bit of grammar - reflexive verbs and more advanced tenses in particular - in them that hasn't been covered at the point when they are offered, which can make them quite frustrating.
Maybe add some more fun units, some less challenging ones, at different points in the tree?
Did really enjoy this one!