After looking over all the whitepapers, this one might be helpful:
Continuous Improvement Process (Written by Chris Fultz)
In addition, looking through some of the posted team handbooks might help for getting an idea regarding how other teams approach involvement in the post/pre-season.

Possibly, poll the members of the team to gauge interest in a development seminar on programming the robot, CAD, animation; though if they weren't caught by the mini-robot build, I'm not sure about the success of this.

My suggestion is in lieu of this, have acompletely non-robot related teambuilding day (or evening). My former team held a teambuilding day on the 341st day of the year- the first time we did this I was worried it would be 'cheesy', but it turned out to be really enjoyable, and afterwards I felt much more connected with the other team members. Hope this helps.