Everywhere will still be open in September.
As has already been said, it will be quieter than the height of the season because the kids are back at school, but still plenty of people around.
You can use ANY pool in Arillas, free of charge. Doesn't matter if you're staying there or not, the owners just ask you to buy some drinks/snacks whilst you're there (DON'T take your own food or drink in - doesn't go down well!)
Lots of excellent tavernas/restaurants (-never had a bad meal yet), and to suit all pockets and tastes. I just tried to count how many and couldn't. You could certainly eat in a different one every day and still not exhaust the list ( - one of the main attractions as far as I'm concerned).
Not a huge number of shops, but most are 'quality', and offer something a little different.
Two main bars, 'Coconut' and 'Malibu' , different atmospheres but equally popular - you choose.
Beautiful sandy beach to the north of the jetty, more pebbly to the south, but safe, warm sea whichever you prefer. You can even go 'au naturel' if that's your thing, at the furthest reaches of both the north and south beach.
If you're feeling energetic and/or adventurous, there's water sports and boat hire available ,although in September you may have to pre-arrange that either through the 'kiosks' on the beach, or through Olgas Travel or Arillas Travel who are also the 'go to' people for excursions and trips and in fact , anything and everything else you might require. Boat trips to the nearby islands are also popular.
Arillas is surrounded by hills and Olive groves, and there are some lovely walks to the nearby villages (Afionas is a must, beautiful village with stunning views).
And then there's the sunsets! 'Can't describe those - you'll just have to wait and see them for yourselves.
Enjoy. You will.