Dusting off old crunchers

I’ve go a couple Core 2 era boxes I’ve failed to dispose of so thinking of putting them to use on something. Just have to decide what! They’re going to be stored in a location with occasional manual internet access. I will want to run a big cache, something like 3 days minimum, maybe 5 to 7. So projects that rely on fast turnaround are out. I would prefer for the project to be more CPU focused since the work done will be insignificant compared to GPU enabled ones.

Any suggestions?