VoxelCode would be a great tool for game jams! Or anything else ready for voxels! It can export to .stl for 3D printing, and .obj for game engines!
The open source code is available on GitHub:https://github.com/kippllo/VoxelCode
And the full website is host there as well:https://kippllo.github.io/VoxelCode/index.html
On the full website you can find the documentation. It fully details the syntax, commands, and features of VoxelCode:https://kippllo.github.io/VoxelCode/html/documentation/doc.html
Can't think of anything to voxel? Check out these examples for inspiration:https://kippllo.github.io/VoxelCode/html/examples/examples.html