Why not just play completed games? They should also have a lot of content, or at the very least the reviews will probably indicate if it's a 2-hour game or 20-hour game. If you're on desktop you can click on "Latest Updates" at the top of the page, then scroll down a bit and on the right side go "Prefix: Engine -> Ren'Py" and "Prefix: Status -> Completed."
Here's a link, probably. You can also sort by rating/likes, or include/exclude certain tags.
I haven't played any good, complete sandbox games so I can't give any recommendations on that front.