Any ideas for a rube gold berg project?

For a my end-of-year science project I'm making a chain reaction thing. I can't think of anything that would actually work out for me. So if you have any ideas on what I can do (and use things that can be found at my house). I have a 6 year old sister, so I have a bunch of toys I can use, and I have stings to use too. My goal is to get a Rapunzel doll to her high tower, like the carriage would be pushed to it. Thanks:)