Solo breaks are great. I'm in my late 60s now and have done this a lot over the years. I enjoy being with someone on hols, but I absolutely love being on my own too. It is so liberating, and a lot easier than I thought the first time I did it. I was a lot younger then and I was far more shy than I am now, but it worked out well.
As for places to go, well the world's your oyster really. There are so many places around Europe that are so interesting, lively, sunny and warm that you are spoilt for choice. Others will have suggestions. I've been to the Basque country Bilbao, San Sebastian, and Bayonne, and it was amazing. The food OMG. Another nice place (to me) was Nice and surrounding areas, also the seaside town of Ostia Lido which is 25 minutes by train to central Rome, that was brilliant for a solo, as I could see Rome by day, and flop on my balcony by the sea in the evenings.
Anyway, for me I found the most difficult thing was eating dinner at night alone. It might not bother others, but I personally felt awkward amongst the couples and groups. Perhaps I shouldn't have, but I don't believe in doing something because others find it easy! So my solution was to have a gorgeous lunch which I found far less formal than evening dining, and after a day wandering and sightseeing I had a hotel room picnic on my balcony. Worked for me.
I am older and invisible now which is great ha ha, so I had no hassle from cheeky men swooping around either. Perfect 😊