In terms of stuff you do yourself, you could look at Project Euler and Advent of Code for interesting programming problems that stress efficient solutions and testing. There are a fair number of solutions for previous Advent of Code, so once youve cracked the problem, have a look at how others solved it. You;d learn a fair bit. Similarly there are cipher challenges (Alan Turing - manchester, ...) which are interesting.
Plenty of previous maths competitions (ukmt, liverpool, lincoln, mathsbombe, ..). Similarly, there are many pop maths books as well as things like Tony Gardners "The Essence of Mathematics Through Elementary Problems", lots of recreational maths books by Martin Gardiner. You could have a read through a greek/history of maths book, ... Look into polyogonal numbers, collatz sequence, ... Depends really what interests you.