All the advice about productivity ignores the process of getting good ideas in the first place. We need to have smart ideas about how to become smarter.
Here are a few things:
Explore other languages. I don't mean only "foreign" language, but also things like mathematics, or music, or visual arts. When you think of aspirations to be inter-disciplinary, think of those other disciplines as disciplines, not fields. (Fields are like other territories, different places. Disciplines are intellectual approaches to doing things.) You can explore other fields too, of course.
Commit to it. Whatever "it" is in this particular context. An example might be my blindfolded rhythm changes. It's just a different level of commitment to something that might be unimaginable to many people. It might be reading more than the next person, or really learning something well that most people just learn in a half-assed way.
Catch people in the act of being brilliant. If you aren't doing this, then you missing out on a lot. You should be able to explain how and why something is brilliant and be in absolute awe when it happens. So many times I've been amazed when students can read something brilliant and be unimpressed by it. A lot of times the undergraduate students will be more brilliant than the grad students, even when they are taking Spanish for some non-literary reason. You can catch a student being brilliant from time to time. I am difficult to impress, but I am still amazed every day by some piece of human creativity.
Catch yourself being brilliant. This one is a little tricky, but it is hard to cultivate your intelligence if you can't identify moments when good things happen, so that you can reproduce that phenomenon again. For example, it might be two ideas that weren't connected to each other come together in your mind, so that you see them together in a relation rather than as two separate ideas. Or you might realize that you have been using the same word about two very different things, but without realizing they were different.