In my classroom, we used to participate in the Math Forum from Drexel University. Students worked in groups to come up with answers and viable explanations to complex Math problems. Then, Math graduate students and professors would write back if their answers were incorrect with tips or questions to help them solve correctly. At the end of the week, they would choose students to be on a gold or silver list based on their correct answers and explanations.
While we only subscribed for 2 years, it was a very valuable program. You could definitely try something like this in your classroom…as a Math center, or even just as an activity every other week or so. Students work in teams to come up with an answer and an explanation to a problem, and then you could choose the “gold” answer and the “silver” answer to be celebrated. It would be great way to practice problem solving…and if you are strapped for time and would like me to judge your Math forum, I am more than happy to help! 🙂
Here is the site for examples of old problems.
Here is another good resource for grades K-8 problem solving problems/answers. These are arranged by grade level and standard/topic. They would be excellent centers for your Math Workshop!