A plethora of maths problems offering varying degrees of challenge
from whiterosemaths.com
You may also enjoy the sumaze app for your phone or tablet. You need to download it from your app store but it is free.
Post-16 maths problems, articles, advice and features
Every Thursday at 17:00 (UK time), they livestream one hour of maths problems, puzzles, mini-lectures, and Q&A. It’s free, interactive, casual, and relaxed, and the livestreams are recorded so that they can be watched back afterwards.