Dear George ,
Interesting question , I must say!
Here are few suggestions for Pre-Onboarding :
Send the material*, for Induction , announcing a test on them on the Induction Day. This is just to ensure that they read the material and gain an understanding of it.
Share the training program outline, with certain study material , so that they can work within their limits and are ready for it, once it commences.
Connect them to your employees through LinkedIn and Social ERP , so that they can start networking. Form a LinkedIn group to initiate discussion and interaction between them and your existing employee base.
Complete the Medical Test before Induction, so that you don't have to relieve them , once they report to you.
You can even schedule call with them creating a number to bridge the conference call, to keep a tab on their progress.
A walk-through your Office Premise would save their time once they join in.
Finally , if only you have the option to, consider an internal Elance for them. Involve them in small pet-projects , where they can get a feel of the work, understand their potential and learn more about the working culture of the firm.
I am certain there are more ways to involve them productively , before they join in for a full time role. Looking forward to hear from our experts.
* [including the Employee Handbook, Process document for managing the ERP to raise a request for leave or reimbursement and so on]