I second Javascript. It is widely and immediately available (even gui comes for free), allows using and learning different styles of programming, can be used to

I second Javascript. It is widely and immediately available (even gui comes for free), allows using and learning different styles of programming, can be used to immediately write useful little programs, and scales up to moderately sized projects. Python is good too. Not always immediately available, but easy to install. Other than gui (which would need to be learned), shares many advantages of Javascript.