Another way to put this is, what tools exist to generate sentences in the same way that some conlanging tools generate phonotactically valid words.
I've heard people suggest using word generators to generate sentences. I'm open to the idea, but it seems that morphology and syntax aren't close enough and I'm not smart enough to put my finger on why. If there is a word generator that can do sentences, I'd be interested in looking at it.
Here is an example I laboriously wrote for Toki Pona.
Note: I'm well aware there are a bunch of tools for just generating a random sampling of a dictionary, but that doesn't create syntactically valid sentences.