Read read read!
Read the a level textbooks for a few subjects.
Do an open uni course.
Watch educational YouTube videos.
Watch ted talks.
Read the non fiction bestsellers (non celebrity)
Read broadsheets.
Read political magazines eg economist/ spectator/ prospect.
Read other mags like national geographic or history/ other obscure ones.
Buy an atlas and encyclopaedia and read both cover to cover.
When reading a book by someone who likes to use lots of big fancy words (eg Steven pinker) keep a notebook to hand and write down any words you don’t know the meaning of or don’t use a lot and keep this book with the dictionary definition somewhere you can reread it regularly.
Write letters- it’s a good way of practicing your writing and thinking skills.
Keep up to date with what’s happening politically - nationally and locally. ie read consultation documents and respond to them, start/ respond to petitions
Watch documentaries eg look up the big list on iplayer or their content on bbc4. I really like their archive films.
Go to charity shops in upmarket/ trendy areas or next to a uni. Have a browse through the different non fiction and academic subjects.
For history it can be easier to start with historical biographies rather than straight history books.
If you are thinking of reading literature then I’d recommend getting the study guides to go with the book or get some general books on literary criticism.
Or you could cheat and get a book of summaries of all the best known novels etc!