Programming for children

My daughter who is in primary school has an assignment to write a program to calculate the dates of Easter for any given year. A programming language to use has not been specified. What language should I teach her to use? She's got three weeks.

As a bonus, it will be extremely cool if it is something which she can take into school on a USB stick and actually run a demo without having to install anything on the school machines. All her school machines are Windows machines.