Venus shows changes in angular size and also shows phases similar to those of the moon. when galileo first saw these changes, he concluded that

Venus must orbit the sun...
Until then everyone thought that all the planets orbit the earth. 
But Galileo realized that this isn't true unless, like the moon,
(which actually does orbit the Earth) its size remained the same. 

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.