Magaluf - Majorca
Ayia Napa - Cyprus
Faliraki - Greece
Play de Las Americas - Tenerife
San Antonio - Ibiza

Mid Septmeber the Med should still be warm and resonably busy but maybe not as full on as the Height of the season.

Kind Regards
Stewart

Albufeira - The Algarve......the new town is a great place for a group holiday, plenty of nightlife and the weather will still be hot.

Benidorm - a group of about 40 of us (all blokes) went there early June to celebrate a mates birthday - great laugh.

Also Bodrum, Turkey and Kavos, Corfu. Take your pick.

Most of them mentioned are full-on 18-30 holiday type of resorts.
Going by your username they would probably suit you to a T.
My son has been to Magalluf, Playa de las Americas and Kavos with a group of mates and is in Turkey at the moment. I'm sure you'll have a great time wherever you go