I am new to game development, and currently working on development a mobile 2D game (for android). As the part of the development process, I need to build a prototype and playtest it, to see if the game mechanics and user interaction is ok For example: if I have a starship shooting at ememies, I need to see

• what's the best size for my starship.
• what trajectories should the enemy ships fly and what velocity.
• Should the enemy ships be coming only from left to right, or also from top
• Should the enemy ships form a 'flock' or just fly by themselves
• what's the best 'powerup' pickup mechanics: to shoot it, or to pick it with the ship etc

Implementing these details directly in Java (Android) is time consuming and as many of the 'hypotheses' will be rejected, I also don't want to invest a lot of time to code thigs, majority of which gonna be rejected.

I found 'tool' Etoys http://www.youtube.com/watch?v=34cWCnLC5nM&feature=related and official website http://www.squeakland.org/ which helps to build 'prototype' quickly, but Etoys is meant for children learning programming and is too basic.

SO MY QUESTION IS: Is there any prototyping tool, as simple as Etoys and with better prototype quality?