Weather late November can vary quite a bit. Really hard to give a prediction because that is the transitional period from wet to dry seasons. If rainy seasons ends early (started late this year) you might not see much rain and normal temperatures (65-85). If rainy season is late ending, it could be quite wet and a bit colder (55-75). That is also the time of year the winds start to kick up but that isn't usually an issue until Dec/Jan when a cold wind will cut right through you. Either way, it shouldn't be too bad and might be very nice. I will guarantee there won't be any snow
As for Thanksgiving, that is another American holiday that has no influence on the locals so action is normal to good because of the influx of visitors. Sportsmen's Lodge has a GREAT Thanksgiving dinner with turkey and all the trimmings not to mention the Dallas and Detroit vs. whoever football games on the TVs. There will be a LOT of ex-pats around that day since we can't find a real turkey dinner hardly anywhere else in Costa Rica...