1) Guitar: Solo Lite
This is really cool app. Very easy to use and the feel of the strings are amazing. The output is also pretty loud so that you can entertain the crowd (if u knows basics of guitarJ). Also this is one of the most popular app in Market.
2) My Piano
This is another awesome app and easier than guitar with great sound
Just change the setting and it will act like Mouth organ, Saxophone, Mixer etc
3) Google Sky Map
This app will help you to explore the Sky. Yes Stars, Galaxies, Planets etc. All those, which have fascinated you in the childhood. You can check what you see on the sky is a planet or a star or a galaxy. This is may not be useful but fascinating app.
4) Google Translate
You can translate words and phrases between more than 60 languages. Also it supports voice input for some languages. Also you can hear the translated output.
5) AndroZip File Manager
This app will help you to browse your SD card. This works as File Manager, Archive Manager, App backup. Also you can search files and send to other devices.