I would start with listening to podcasts - science, news, lifestyle topics each time you drive somewhere.
They are incredibly interesting and you can change between different subjects until you find something for you. It’s free.
Then download an app like Blinkist - the free version. You get a free daily non-fiction ebook ( 15 minute summary of it) which you can either listen to or read.
Watch documentaries- set yourself a goal for example two a week to start with , look for something that actually interests you. You might find that you discover interest in completely new subjects.
Find one newspaper ( online copy) that is comfortable for you to read ( they all have different styles) and spend 15 minutes in the morning reading a few articles that interest you. It’s not about absorbing all the headlines - it’s better to digest one or two subjects in depth and reflect on the views raised and what your own opinion on the matter is. It stimulates thinking.
Listening to Ted talks is also great and really entertaining. And free.
If you discover a subject that really interests you , go and do a you tube search on it- thousands of free materials to enhance your knowledge.
Talk to people who have different opinions to you and debate subjects. Don’t be afraid to think outside the box and look for alternatives.
I know general knowledge is not the same as intelligence. But the more you stimulate your brain to think the more ‘intelligent’ you might feel you become , it’s like physical exercise but for your mind.