When I work with advanced or talented and gifted students, I recommend that enrichment be used to “go wide, not deep.” The point is not to simply move the students vertically, but challenge them to go deeper with current content or use critical thinking activities.
Here is an online resource of problems from the Paul Erdős International Math Challenge that will indeed challenge almost any student (check the problem below for 3rd and 4th graders).
Here is another online resource of problems from the Art of Problem Solving (AoPS). The link is to a search posts that address practice problems related to the American Mathematics Competition for 8, 10, 12. Below is an AMC 8 practice problem.