fun things to do in winter with kids aged 8 and 9?

So my nephew and niece (in law family) are coming down to Ky for the first time in 6 years! Their dad has been withholding them from their mother (my SIL) and now thanks to court and lots of lawyer fees it's all straightened out. We will be staying with my SIL from Dec 14th - Jan 8th since we live 3 hours away and the semester will be over. The kids will be down dec 24th-Jan 1st. What are some cool things we could take them to do, or projects we could do inside?