1. Why do they have to be "junior"-level? When you start a job, do you want to be doing easy stuff? Or do you want to be doing the fun, challenging stuff?
If you're looking for projects that have bugs that are easy, or easy-but-tedious-and-time-consuming, you probably won't find very many. The easy bugs tend to just get fixed, the tedious ones? You don't want to work on those.
2. I've always wondered how far people get with other people saying "contribute to this project because I like it!" — at the end of the day, after you've spent the past hours working on something else, are you really going to spend the next hour coding for something you don't care about? Or are you going to say "meh" and play Starcraft?
There must be some app/tool/… that you use that is open source? Thats a good ones to look into contributing to. Because you already use it, so its easier to care about it.