Can anyone tell me about the salt baths or thermal baths in Budapest?
Plenty to tell, what specifically do you want to know? The Rick Steves Budapest and Eastern Europe guidebooks have extensive detail about how to visit and use the facilities at the larger baths.
Many years ago I went to the Gellert Baths in Budapest and loved it. www.gellertbaths.com Just google Budapest baths and you will overload on info then come back and ask for thoughts on the ones that appeal to you.