At ULearn, I was shown how to set up Conditional Access Activities. Basically, the students cannot access certain activities or resources until they have met certain criteria in earlier tasks. This kind of flies in the face of what I want to do with allowing students to find their own pathway to learn the key specific learning objectives for each topic, but it has some huge positives!
Check out this good overview. It has great ideas for providing support material (for struggling students) and extension work (for the high-fliers).
I can see the students seeing this as a kind of 'game', competing to get further through the "course". The way I see my extension students compete with each other on BestChoice (see below), I can imagine this may have real success.
Also, there are some game elements out there (for free) which could be made to pop up as they reach certain achievements. Look up "Achievement Generator" in a search engine and you find things like this:
Yes, it is going to be a lot of work, but it will only need "tweaking" every year... As a well-respected educator said recently, "Don't wait for it to be perfect to start, just start doing it. Otherwise you never will..."