Project Euler is a massive collection of cool problems that combine math and coding. They're pretty fun to solve and I enjoy writing up my solutions and benchmarking them, especially on much larger inputs. Sometimes I also like to tackle more general problems than the ones presented.
I solve the problems using Julia and benchmark code using the BenchmarkTools.jl package.
Project Euler strongly discourages the sharing or publishing of any solutions beyond the first 100 problems. I disagree with this stance though. I think nobody should be sharing or publishing the numerical answers so that others can just copy paste without putting in any effort or learning anything. But I also think we should be encouraging high-quality explanations and derivations, good quality code, and explorations beyond the original problems. I do learn a lot by getting stuck and struggling through problems (some of which I keep revisiting for years) but I also learn a ton from others who share their solutions.