If a non-US citizen (EU national) is married a US citizen, is there any way for her to spend large amounts of time in the US without becoming a permanent resident (if she decides not to seek any kind of employment in the US etc.) ?

No.. Unless you are in US with different status such as employment visa or student visa.

I believe one can apply for a B-visa, which is a long term travel visa. I think she can get approved for up to 6 months and maybe even longer, but she cannot work.