Where to go on my own next May. I like...

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A nice beach

Quiet (preferably adult only) hotels where I can chill by the pool but not a million miles away from the markets / shops

Few nice places to eat

In Europe

Shopping (not designer just random browsing etc)

Maybe scuba diving classes

Turkey ***
Edge of Europe.
4.5 hour flight
Other than that Turkey (specifically Olu Deniz) definitely fits your brief. Lots of all inclusive +/- adult only. perfectly safe for a single person. (I've been going on my own for well over 10 years and feel far safer there than in the UK.)

Lots of trips and I did a trial scuba a few years ago which was circa £25, inc pickup and dropoff from and to hotel, lunch and 2 trial dives. (They do the training for quals too). Loads of interesting trips be it history, geography or just pure fun.

Loads of shops to wander round and if you want 'genuine fakes' they abound by the thousand from the difficult to tell apart to the total fakes. Great fun and the most lovely people

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Pretty much any resort in Greece fits the bill - so narrow it down based on budget, ease of getting there, type of beach…

I'd second Turkey .... any of the Coastal Resorts really. Fethiye is lovely but the town itself doesn't have a beach. Weather should be ok....
Valencia is lovely...beach and town not too far from each other...not sure about the diving though.

Antibes - fly into Nice.

Sardinia

I have never been but a friend just booked it and I want to go too!!

I love Spain for clothes shopping. I actually buy more new clothes there than I do here - I'm not a big shopper
I'm planning on going to Massimo Dutti in Palma in a couple of weeks - none of these anywhere near me. Many big resorts have Zara Mango Stradivarius bershka and Pull and Bear. And I love Desigual
So it would be Spain for me at an adults only hotel