I have always worked for product companies, on products that I find interesting. Currently it is a product for margin calls in finance, before it was SMS routing and delivery, before that VoIP. Even if everything I do isn't new or interesting, there is enough variations and challenges, both in the code and also learning more about the domain. The times I have started to feel that it was boring and repetitive, I have changed jobs (but I've stayed at least 5 years in each place).

I also like to try to get better at what I do as a developer. Mostly this is by reading books or taking MOOC courses. I think there is quite a lot to learn about developing SW well (because it is a really complex activity). So that also keeps me interested.

About eight years ago I also started blogging [1] about SW development. I've found that trying to formulate what I think about it has also kept it interesting.

[1] https://henrikwarne.com/

Thanks for sharing. I'm currently doing fullstack web development and find it not as interesting anymore as it used to be (close to 6 years now). I'm trying out various projects but it is either need higher learning curve such as deep Math (i.e., machine learning, graphics) or domain specific knowledge which I usually don't have expertise about.

I guess it is part of the learning.