Both the Four Seasons and the St Regis have a superb spa and have a sauna and steam set up as well that is available for all guests.
As for Aer, it was very chic and cool a few years ago but honestly it’s not worth it beyond the views. The St Regis is taller and has a bar and a restaurant on its top floor as well. Beware though that both places are major party spots on the weekend and the Regis brings in external security/bouncers that are unnecessarily intimidating and aggressive. This, along with the fact that the rooms feel a bit dated, is why I prefer the Four Seasons over the St Regis.
In any case, there are way better cocktail spots in the city than either hotel’s bars (Otra, Americano and The Bombay Canteen are primarily known got their food but serve incredible cocktails). If you’re even a little bit doubtful, I would avoid the street food altogether. Juhu not worth it, unless you have a particular place to go to there. Bandra has a bunch of walkable areas with some interesting food, bars and cafes so definitely worth it more than Juhu.