My daughter(now 12-3/4)was very similar, we had boxes of "stuff", like rocks and strings and little things she would make from sticks and grass, and that sort of thing. What we tried is to put things in plastic storage bags and "save" them in a particular spot in the garage. She had her own special "storage" area, and we would pack away for later, anything she valued.Anything. Then, I would get a big garbage bag and put all the little bags of "stuff" in there, according to what it was. All the grass, sticks, stones,tinfoil shapes, etc. would go in 1 bag, and the old McDonalds happy meal toys would go into another bag all it's own, older stuffed animals into another yet, and so on and so forth.She could go out there and play with them anytime she had playtime, but had to put it all away when she was done. That was sort of a struggle at first, but then she started playing with the "junk" less and less. She would get out the stuffed animals more, and after keeping it all for almost a year (have to think long term here) I took out the bag of "junk" from the garage, asked her about it, and she hadn't even noticed. It kept the stuff out of her room after about a week, cleared the area, organized the stuff, and helped her get rid of some low priority things. We made sure to ask, after a long time, if we should just dump the bag of junk, and when she took a look in the bag, she was surprised she even had wanted to keep it.We had a rule after awhile, that if she got something new, she had to take something similar to the Save Spot bags. She made the decision as to which object went out into the garage. I never argued with her choices. I didn't always agree with them, but the stuff was hers, not mine. If she got rid of something I thought was valuable, I 'd save it special, so we didn't toss it.
Good luck, it can take a lot of time and work!