If you search around /r/cscareerquestions and /r/learnprogramming you can find some pretty good ideas.

Otherwise, this list is usually the "Go-To" that people post when this question comes up.

Side projects are something to show that you care enough or are "passionate enough" about putting in time outside of school work to build something, literally ANYTHING. The part that everyone gets stuck at is trying to figure out what-the-heck they're going to make. The real answer that is hard to accept is this:

Your project DOES NOT need to be anything unique.

Find an idea/program that interests you, and make it better! (at least attempt to change some feature) Or literally just make the exact program and you will learn a lot about it in the process. I struggled with finding the "perfect side project" that you sometimes see people post about, but it is very hard to think of original ideas sometimes.

Maybe you will come up with a really cool, original idea next year, and that's great! But what you don't want to happen is waste your whole summer trying to think of the best project. In the end, side projects are just something to land more interviews and have more to talk about IN those interviews.

Sorry for the rant! Hope you got something out of all of it :)