Khan Academy for Kids - Two Thumbs Up

It's a delightful app which includes games and puzzles.  It's for kids in preschool and perhaps a bit beyond.

When you do well, you get to give surprises to the animal friends, which itself is also just lovely.

My 3 year old loves this as one of his favorite apps / games, and one of his favorite ways to learn.  It really holds my son's interest and he learns a ton. 

Google Play store:
https://play.google.com/store/apps/details?id=org.khankids.android