Can I give my rabbit string?

I would like to get my rabbit more toys, but at the moment I can't just drive to the pet store. Considering that I'm only 14. But I would like to know if I could give my two rabbits some string? Would it hurt them? What I would do is tie the string to the top of the rabbit cage and just have the string hang down. would that be harmless to my rabbits? Please help me, and thank you for your time on answering this silly question!