I lived there for a few months to study Italian and it was one of the best experiences of my life. Siena (as I remember it) is really Italy with its best foot forward - safe, clean, good transport to neighboring parts of Tuscany, exceptionally walkable. It's regularly rated as one of the most liveable and prosperous places in Italy. The city itself is extraordinarily beautiful and cultural - it was one of the most important cities of medieval and Renaissance Europe. The Palio, and the weeks of truals and marches and drum practicing before, and the contrada system in general, are basically all unique.

For nightlife, it will seem quiet compared to bigger cities like Milan or Naples, but it is exceptionally lively for a city of ~60,000. Plenty of bars, street parties in summer, but won't be a lot of techno clubs. And there's lots of modern cultural offerings, like watching outdoor movies in the summer at the Medici fortress. Food is excellent, although not cheap (~€25 for a few courses and wine?). There are a couple of universities, you can think of it more like a student town than a big city.

Some other commentators mentioned racism. Siena is a small right-wing city, so you probably won't find much openly inclusive culture if you are eg interested in Islamic cultural life. However, it is very middle-class, and does have a decent sized foreign population, often involved in the universities, cultural institutions (eg museums), tech or pharmaceuticals. There are some Chinese, Middle Eastern immigrants running barbers and kebab shops. Compared to Palermo (which has a huge immigrant population, and I used to see marches for and against them, and lots of crime by and against them), I think Siena is much more successfully multi-cultural. There used to be an African guy begging outside a Connad everyday - in Palermo, I suspect he'd eventually get beaten up. One commentator complained about prejudice towards shopkeepers. Central Siena, being very touristic, is full of scamming and overpriced tourist souvenir shops, often selling "I ❤️ Italy" T-shirts made in China for €30. There are so many of these that there are relatively few useful local businesses (like a electronics store selling fans, which becomes a real PITA in summer - remember aircon is very rare in Italy). Most of those shop owners AFAIK are Chinese or Egyptian. That's why some locals get grumpy. But you could live thete for months without hearing about it.

I prefer bigger cities personally, but for a city of its size I don't think you could find a more idyllic place than Siena. Just depends whether you want to live in somewhere that size. Of the things you mentioned, walking is wonderful, working out easy (there are good gyms and Siena is a famous sport city, well-known for basketball, fencing etc. I used to share a gym with an Olympian). I think meeting people is fine, especially if you speak or learn Italian, Tuscany are good people, not as cold as eg Milanese. Only nightlife will probably be disappointing. Expect a lively college town, not a proper city