A really mini H-drive probably as small as it can get.
Here are some photos:
And a video:
We made an even smaller one a few years back by using the special 24-tooth gears to put the sliding motor directly above the drive so it didn’t stick out the back. Impractical? Yes. Adorable in some weird niche way? Also yes.
This robot is incredibly cute… Amazing job with getting the side omni-wheels to almost mesh their wheels.