I think what you need is inspiration, there’s so many things to build to improve your skills and broaden your knowledge. Not every project you work on needs to be novel and exciting, even basic things can provide ample complexity to learn new things.

For example, in c++ you can make a basic image viewer that takes the name of a file and displays it, using some library like SDL (or OpenGl/Vulkan(hard)). This would teach you a lot about file formats, dynamic memory allocation and managing it, graphics pipeline etc.

For html and css, you can try to recreate some cool websites you see online. I hope this gives an inspiration(best viewed on PC).

For problem solving there’s always leet code, advent of code and other similar services.

Here’s more stuff to get started

You have 5443200 seconds to make your choice.

Though you give me pain and suffering, C you will always be my beloved.