I study physics and computer science and both studies include minor programming tasks whereas larger tasks are only (potentially) a part of larger projects like for your thesis. When I look at job postings for scientific jobs e.g. in the area of HPC they usually ask for "extensive programming skills in C/C++ and HPC" or the like. This is what I would like to achieve, but frankly I don't know how.

I don't mean to mourn, but you can imagine that studying physics/CS means having a busy schedule so I couldn't even work part time as a research assistant to get practice without negative influence on my grades. The second problem is that I lack ideas on what to code in my spare time. I would like to do something useful but even for open source projects you apparently need a whole lot of time and depending on the type of software a lot of experience to be useful.

I would be grateful for any advice you can give me.