Here are some interesting math extension activities for upper grades:
Games from our 3-5 Games Page
Grade-level aligned games from Greg Tang Math
What’s Going On in This Graph? A monthly activity from the New York Times that focuses on analyzing visual data.
Girls Adventures in Math Sample GAIM math competition problems. These are engaging, challenging problems that have different solution paths. Try them out!
YouCubed Math Task Archive Many engaging, fun math tasks - filter by grade or just pick one that looks challenging and fun!
OrigamiWay Symmetry, fractions, shapes - it all comes together when you make origami!