We've just spent a couple of weeks building half-timbered houses out of foamcore and coffee-stirrers.
So a fairly easy project might be a good way to get them started.
Last year we spent several weeks (seemed like a whole term at the time) working on pirate ships using a slightly modified version of the Gary Chalk ship plans, modified because we used foamcore (no budget for balsa), but all seemed to enjoy it and we now have a huge flotilla of ships in various shapes and sizes.
Might be worth doing a google search for the Gary Chalk plans to save yourself some of the work...