The phrase pattern
one something would be plenty
means one of something would be the same as having many (plenty) of something
when one has plenty of something, one is left without wanting more
It can have two different meanings that the speaker is saying the one of something is very rich and dense or they could be saying that it bores them and they don't really want that much of it, context is very important to figure out the really meaning. It depends if one is a little or alot.
eating one banana split would be plenty
after eating a banana split I will be full
one drink would be plenty
after one drink I will be drunk
In your original sentence, she might have used
I think going for only one day, would be plenty
I think going for only one day, would be enough
She is basically saying to you that she only wants to go for one day and by setting her conditions out, if you're willing to abide by them, she wants to go with you.