September is fall in the northern hemisphere which is a lovely time to organise a vacation getaway.
It's still beautiful weather in most of Europe.
Southeast Asia is a good destination too as it's not a big rain month there and it's less busy.
Europe still has a lot of options for you in September.
If you are in the UK then there are still a fair few retreats available and you don't need to jump on a plane.
Bali is a clever retreat spot at this time of year. It' still good weather but less crowds.
Try out a Bali health retreat maybe, if you are looking for a holistic time away.