I have recommended Nrich who publish new problems to really make you think every month on the Challenges page here. The site has been updated recently and now has a great new look and is easy to navigate. There is a clear guide for students. You can sign up for an Nrich student newsletter if you want to be notified of new developments on the site. If you have not tried Nrich problems before you may find their recommended starter problems good to try.
On the new site just select your chosen home under the Student list. You could then for example search for activities by curriculum topic, for example look at this collection for Lower Secondary. (Note the collections available on the top right of each student home).
Why not try a problem and send in your solution to NRICH?