You can try to write programs you think would be a challenge for you (but still doable at your skill level). Write some games, bots that solve simple games like Tic Tac Toe or mods for your favourite (moddable) games. If you own a Raspberry Pi you could play around with the DPIO. Your free time projects are usually nothing too exciting or world-changing and that’s perfectly fine, they don’t have to be; the worthwhile part about them will be the practice they’ll offer.
Alternatively, you could try reading some theory, learn different types of data structures, sorting algorithms or pathfinding algorithms and their respective strengths and weaknesses. Or go deeper and prepare for higher Edu in CS by looking into the maths fundamentals, learn some linear algebra, discrete math, analysis or basic graph theory, it will help you a lot.
Edit: graph theory, not knot theory
That is a very specific subset