Resources depending on prep time:
- NRICH activities and games: https://nrich.maths.org/primary
- Peter Liljedahl numeracy tasks: http://www.peterliljedahl.com/teachers/numeracy-tasks. Despite the dull name, the activities are good for catalyzing interesting discussions. His card trick videos were really good with my sons: http://www.peterliljedahl.com/teachers/card-tricks
- We’ve gotten a lot of mileage from the Beast Academy books: https://beastacademy.com/books. The problems are thoughtful and there’s usually one or two problems in each block that is a good inspiration for a conversation or deeper exploration.
- More prep time, in increasing order of advanced time required
- http://mathpickle.com/: the puzzles and games are very good.
- Math Teachers at Play blog carnivals: https://denisegaskins.com/mtap/. Variable amounts of prep time, but usually there’s at least one activity that is ripe for exploration, may take a bit of reading through the carnival to find a suitable one.
- Mike's Lawler's blog: wonderful collection of (mostly) videos of his family working through problems, puzzles and mathematical explorations. Because his kids are older, it will take a little time to find something you think is suited for your son and then a bit to organize the activity.
- Georgia State math standards: Despite the name "standards," these documents have a full curriculum with a collection of really great activities. As with any full curriculum, not everything is a complete winner, but there are enough gems. Also, this is probably the best resource for finding material to complement a kid's weaknesses.
This page is still the most comprehensive list of our favorite resources: http://3jlearneng.blogspot.com/p/favorite-educational-resources.html
Unfortunately, it is a little dated as I haven’t really been maintaining this blog in the last 2 years.