You will find very few places on SXM that will NOT be open air. If you hadn't said September, I would have suggested perhaps Orient or Grand Case, but traditionally many places are closed on the French side during that time. It's possible that many restaurants will not take their vacation then, due to just ramping back up after Covid, but who knows? Otherwise, probably your best bet is Simpson Bay, which is different than Cane Garden, as it's a lot bigger overall and while there are many places you can walk to, for the most part, it is not a 'toes in the sand' kind of walk, more a 'walking in the road' kinda walk. SXM is overall busier than the BVI. Do you have a place to stay?