Which erodes faster-limestone, sandstone, or magma rocks?

I'm pretty sure its limestone, if not, then its sandstone

Depends on the erosion factors... If the factor is an acid rain then the limestone is the first one. If  the factor are wind and water, then the sandstone is the first one. In any case scenario, the magma rocks are the last, it is difficult to erode  

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.