Games like Unserious Sandbox
Games like Unserious Sandbox include Garry's Mod, Clustertruck, People Playground, Refunct, and Goat Simulator. We matched them by the tags players give Unserious Sandbox on Steam (Sandbox, Parkour, Physics, Abstract, and 3D Platformer): closest matches first, and among equally close ones the better-known games.

Garry's Mod
Shared: Sandbox · Physics · Casual

Clustertruck
Shared: Parkour · 3D Platformer · Physics

People Playground
Shared: Sandbox · Physics · Singleplayer

Refunct
Shared: Parkour · 3D Platformer · Relaxing

Goat Simulator
Shared: Sandbox · Physics · Casual

Totally Accurate Battle Simulator
Shared: Sandbox · Physics · Singleplayer

Zup! S
Shared: Physics · Casual · Minimalist

Townscaper
Shared: Sandbox · Relaxing · Casual

Noita
Shared: Physics · Sandbox · Difficult

BONEWORKS
Shared: Physics · Sandbox · Singleplayer

Teardown
Shared: Sandbox · Physics · Singleplayer

Kerbal Space Program
Shared: Sandbox · Physics · Simulation

BeamNG.drive
Shared: Physics · Sandbox · Simulation

Universe Sandbox
Shared: Sandbox · Physics · Relaxing

Human Fall Flat
Shared: Physics · Sandbox · Parkour

House Flipper
Shared: Sandbox · Singleplayer · Casual

Hook
Shared: Casual · Abstract · Minimalist

Getting Over It with Bennett Foddy
Shared: Physics · Difficult · Singleplayer

Poly Bridge
Shared: Physics · Sandbox · Singleplayer

DrainSim
Shared: Sandbox · Physics · Casual

Space Engineers
Shared: Sandbox · Physics · Simulation

Superflight
Shared: Casual · Relaxing · Physics

GoreBox
Shared: Sandbox · Physics · Casual

Overgrowth
Shared: Parkour · Sandbox · Physics