One of the best investments is undoubtedly in yourself, your knowledge and experiences.
Spend money to grow in your career by going to conferences, taking courses, reading books, or something else specific to your job. It may feel like you don’t want to spend the money on something like this, but if it pushes you outside your comfort zone and causes you to advance in your career, it’s usually worth the money.
There's always something new to learn, especially talking about your career or your job if we see it from the perspective of technology, there are always innovations that serve as excellent tools to optimize and even facilitate your work. It's great to be up to all these advances, even if your career doesn't belong to some technological area, nowadays anything goes hand in hand with that.
So there will always be learning methods as options for you to invest in, and the best of all, is that it doesn't have to cost a lot of money, there are great options, you only have to do a good internet search of what you are looking for.
Udemy is one of the most complete options, it's an excellent platform for online courses. Don't doubt to try it.