It's very context sensitive. "That's nice" (with at least an implied exclamation point) can be a literal reaction to something you like. "Yeah, that's nice" as a reaction to a statement/argument can also be the equivalent of "Whatever." i.e. it's stupid but I'm not even going to dignify it with an explanation of why it's stupid.

What if it really is actually nice?

"How nice!" is a positive exclamation with minimal wording change from "that's nice".

You can also be explicit about who's being nice. "That's nice of you."

This really depends on what you want to mean by "nice".

Then you have to struggle to find another word to say that it's nice, remembering that saying "it's interesting" is forbidden, too.