Downtime at a help desk? That's one of those things that I thought happened only in fairy tales.
Down time is when you start finding projects for your team to work on. Some things that I think are a good use of time are:
formal training
research into new products/solutions for upcoming projects
looking at areas that are working but could be done better and architecting a real solution instead of the one that was slapped together quickly
revisiting all those good ideas that you haven't had time to implement
performing general maintenance, updates, and upgrades on servers, workstations, etc
cleaning out and organizing the spare parts, junk, inventory rooms
cataloging and updating software licenses and hardware inventory
If these people can't do any of the above, you may need to hire a little higher class of employee or get them some training so that they can handle it. If these people aren't qualified to do anything extra and you have considerable down time, you've got too many employees for the amount of work that you do.