Holiday World, Indiana Beach, Dollywood and Camden Park are all scheduled to be open by mid June. I think Camden opens this week but I am not sure. Their website is keeping low key about it. they have dates on their calendar but no fanfare on the rest of the site.
All the attractions in Pigeon Forge and Gatlinburg opened this last Weds, at least that is what I have heard. Dollywood has not reopened yet, though. Not until mid-June for Dollywood.