Are these very crowded on weekends in late October?

Can be. The number of short break tourists (esp. from the UK) has risen steadily since Ryanair began serving BB's airport. The spas are more crowded in December, though, due to holiday travelers. Russians are coming back to BB now too.
I suspect the modern Caracalla spa will be more crowded than the traditional Friedrichsbad Roman Irish spa. So if you don't mind going nude, try the Friedrichsbad spa first.
The other option is to go to one of the hotel spas. We usually stay at the Dorint/Sofitel and they have a very nice spa, with a pool, whirlpool, sauna, aromatherapy room and a few other rooms, plus treatments (massage, etc.) as well. I believe their day rate is about 28€. Much nicer locker facilities than Caracalla and they have more relaxation lounge beds than Caracalla as well.
Frankly, we were disappointed with Caracalla the last time we were there. It was full of families on short Ryanair breaks and many parents were letting their kids run wild in the spa--letting them shout in the supposedly "silent" steam rooms and also letting them splash and cause a ruckus in the pools. Wasn't a very relaxing experience.

Tks for the info!

be sure to take the tour of the grand old casino that was in the 1800s a mecca for royals and rich - very ornate interior - tours only before noon i believe before the casino becomes alive again with bettors. Great tour when i took it.

*IF* you happen to be staying at the Dorint/Sofitel for your week-end, they offer an excellent (free) walking tour of town on Saturday mornings (around 11 am, as I recall). Really worth it, lots of good history, interesting stories and even though I've been to BB many times, I still learned quite a bit. The tour guide I had (and I think she's they're regular guide) is fluent in German, Russian and English.

"their" guide, not they're guide, of course.

Typos seem to be a real big deal to some people on this forum but I'm not one of them as I'm only looking for info.

No biggie.

Thanks for the info.