Luanti is a good one too, although it might be closer to a minecraft clone than what you’re describing. It’s open-source though, and supports a fairly large modding community. There are all sorts of user-created extras that you can download and enable/disable as you please, right in the launcher.

They even have user-created game modes, so you can play the more traditional minecraft-style sandbox/survival game, or do various puzzles/mazes/platformers that people designed.