Are you looking only to differentiate in terms of readiness or would you also like to include learner profile and interest. This year I've been making a real effort to differentiate more by both learner profile and interest and finding that students are really enjoying some of the activities. Here is a link to a VoiceThread I made for a creating 21st century curriculum class http://voicethread.com/share/667311/ In it you'll find examples of different activities I did based on all three (readiness, learner profile, and interest) for our numeration study. I'll caution you that I'm much busier in terms of making stuff up to do, but I feel that the kids are reacting better to it than in the past too. Please feel free to contact me if you see anything you like and would like a copy of it or if you'd like things I've done for other units possibly.
I'd also point you to the work of Nanci Smith http://www.e2c2.com/bios.aspx Nanci has some truly wonderful ideas specific to differentiating in math instruction and the best part is that they are actual things that you can do with the kids!
Obviously the issue of grading is a huge one- possibly a separate one itself even. I'm not sure if I want to tackle that right now:)
Overall, I think its great that you are looking for resources and I think it will only benefit your students as you continue to do so.
Good Morning Angela
This year we adopted a new curriculum that has various worksheets available addressing Universal Access. My suggestion is to take a look at your curriculum and see what you can use from there. Also, I use white boards every day in class. This gives me an opportunity to immediately see who is getting it and who needs extra help with the concept. Based on their responses, I can pull a small group to reteach while the rest of the class is working independently. Even though you teach the 80% that are not proficient, you'll always have those who rise above. I like your idea of an inquiry activity to complete after they demonstrate mastery of the concept. Students love working together to solve problems!
I've also had a "three ring circus" approach where I split the class into three groups and rotate them through three different activities over the course of 2 days.
Have you considered using their citizenship grade to reflect work completion in class or homework? For example, an "O" will relfect 90% completion of assignments and an "U" reflects 60% or less.
Hope this helps!