Where's my Water ($3:19)
Problem Solving- gravitational puzzles to challenge and frustrate. Achievable though- even for me!
https://play.google.com/store/apps/details?id=com.disney.WMW
Ten Monkeys (free)
Answer multiplication problems as quickly as possible
https://play.google.com/store/apps/details?id=com.tenmonkeys.multiplication
Skoolbo (free)
Multiplatform (Download Mac or PC, Android or iPad) http://skoolbo.com/ support for literacy and numeracy targeted for 4-10 year olds.
https://play.google.com/store/apps/details?id=com.skoolbo.skoolbocoreskills