I like it already! The style is nice, the UI is fun and friendly.
The level selection is a bit confusing. I got it at the end (because i am super smart! :), just kidding, but since a lot of players only spend a couple of seconds on each game, i suspect especially in a big jam like this, there has to be no doubt whatsoever how to start the game. To me, the level selection looked like a puzzle itself, and i thought i have to jump on the number.
Since you asked, here are some level design ideas:
You could add enemies, touching them restarts the level. The enemies could be static, then you could add other enemies who are subjected to gravity like you are. You could have traps like black holes that kill you but which you can also use to get rid of enemies. You could have objects that you can jump on once you have shifted a jumpable block on the right position. You could have a "puzzle solved" goal that is subject to gravity.
The potential of what you can do with this general game idea is great!!
I would suggest adding only 1 new mechanic per level. A puzzle game can quickly become overwhelming, at which point most player quit the game.
But honestly, if i were you, i would start by adding sounds first, because this would add SOOO much to the fun!
Personally, i use beepbox.co, i am sure it would not take too long to create one music and like 5 sound effects as a start.
Looking forward to your submission!