那就 is similar to "since..., then..."; when it is used in daily conversation, people often wants to give suggestion or solution, as a result of or answer to some precondition (i.e. 你今天不能去, which is often known and omitted) . Besides, 那就 could be briefly and more casually expressed as 那 for the same meaning, namely, 你今天不能去，那以后再说吧。 那 in your example is used to refer to something you have mentioned before.

I think 那 means "then" 就 has the sense of giving suggestions 就以后再说吧 : "let's" talk about it later And the second sentence I think is different in meaning 那 好极了= That is great I think 那 here actually means "that"

It just came to my mind that I think it's good to think 就 as "just/simply" 就以後在說吧： We can just talk about it later Just do it: 就做吧／做就對了 做就對了啊： Just do it! ("Just do it !! Is it that hard? What's so complicated", according to the context, it can be used when you feel annoyed or when you want to cheer someone up)

I'll send you an email about this!:)

thank you!