Don't be afraid to look beyond the grade level you teach. You might find an activity in a grade above that fits perfectly with the unit you are teaching now, or something in another grade level you think might really match the interests of a particular student of yours.
Note... The previous post (below this one) discusses classroom differentiation and opportunities to introduce tasks like these during math lessons.
I will continue to look online and in print for more engaging extensions for your fast-paced and/or easily bored mathematicians! Stay tuned. Here is the list so far:
http://www.louisianabelieves.com/docs/default-source/teacher-toolbox-resources/2014-math-6-8-curriculum-guidebook.pdf?sfvrsn=14 (tasks start on page 25)