The volunteers at Essex use a to do list to great success. One or two people will actually make the list up either throughout the week as projects (for the weekly Wed night sessions) accumulate and/or on Wed afternoon so the volunteers can pick from the list as they enter the shop. The person who usually organizes the list also the one who co-ordinates the various work parties around the shop and elsewhere.
There is also a white-board in the shop displayed quite conspicuously for the regular paid shop employees. The board is divided into different projects or groups of projects-by-category. Things coming up in the next few days, weeks, months go in one area, individual car/loco, and boxes for individual employees are some of the different ways I've seen things categorized, often with all of the above on the board at any given time. (It's a BIG board!)
I think the chalkboard idea
could work nicely for big weekends when many projects would be happening simultaneously. I'd suggest a large (say, 4x5' or "portable classroom size") board placed in the front of the shop near the street-side doors so people coming in from the road can see it upon arrival.
I think page-sized lists would be helpful for ongoing lists when mostly regular people who are already familiar with the shop/campus/operation(s) and could be kept in a place/places easily found.
Ok, that's my input. I'm sure something more refined will percolate from these grounds.