How to inexpensively redo my room this summer?

Well, the most inexpensive way to change the look of a room is to paint it. Don't go too cheap on the paint brand or you may wind up using more than you expect to in order to get an even coating. You will want to go through everything that is currently in your room to A) categorize it (craft stuff in one pile, etc.) and B) decide what you really need to keep and what can go. Once you have done that, you can look to storage solutions. Between the two (paint & storage), you are looking to spend at least $100 - maybe a little more, depending upon how much of what you need.