I've heard this from time to time myself, although not as much with grade 5 teachers since the fifth grade units are so demanding. In Watertown we've tried to get a group of teachers together to develop a bank of tasks for each grade level, but it's a "work in progress" to say the least. Here are some things we've tried. I hope this helps. I'd love to hear of anything anyone else has come up with, as we're all in the same boat.
When the kids are exploring finding the factors of multiples of 100, try to find the number under 100 or under 1000, etc. that has the most factors. What numbers would be likely candidates? Why?
Find all the prime numbers under 1000. Can you write a rule to predict primes?
In Math Thinking... and Building on Numbers... there are opportunities for write challenging number puzzles. Setting requirements for the puzzles such as there must be only 2 correct answers, etc.
Playing Broken Calculator (10 Min. Math) using numbers less than 1.
Investigations of events or places in the building involving data collection and representation. My son's teacher (in Maynard) had a group of kids calulate the volume required to store all the fifth grade desks and chairs while the new middle school is being built, then find a room(s) to actually store the furniture over the summer.
I hope this helps. One thing I sincerely believe is that if you look there is plenty of good enriching and challenging material in the program itself. Going to a "next grade" text book makes parents of bright kids happy, but it isn't in the best interests of the kids to push them vertically. I'd rather see kids digging deeper into what is already part of their program.