Hi Laura
I'm also getting married 19th September, I am getting married in the UK and am struggling to find a honeymoon destination. We looked at Thailand but the weather isn't too good in Sept/Oct. Not sure about Brazil/Argentina.
We are currently looking at Mauritius as the weather seems ok, not too hot though as it is just coming out of their winter. There seems to be things to do during the day and getting around the island seems easy but I'm a bit concerned that it will be too quiet at night time and we want some night life. We are like you in that we want luxury but not to be sat on a beach for a fortnight with little else to do. We've never gone all inclusive before so want to choose a property with some variety at meal times.
There are plenty of amazing spa hotels in Mauritius, a little pricey though so it all depends on your budget. We've been given prices on a few 3* and 4* hotels and have been told that the standards should be higher than that of the normal 3* in Europe.
Good Luck and let me know how you get on.
Jo