Sandbox Mode is secret/hidden feature where you can access a sandbox version of A Modded Adventure.
To enter Sandbox Mode, say ":sandbox" in chat in either public or private. In about 2-1 or so seconds, you'll be teleported to a different game..
You'll be teleported to Sandbox Mode. There is also a, "Stand Changer" at the top left of your screen. If you click it; you can scroll and choose a stand that's even unobtainable, for example, you can choose the unobtainable Stand, Queen Crimson.
Note: It seems that there are only 1 player servers, friends cannot join you.